Default to C.UTF-8 locale and deprecate /etc/debootstrap/locale.gen, DEFAULT_LOCALES, LOCALES + DEFAULT_LANGUAGE (WIP)

Closes: #125

SC2181 = See if you can use ${variable//search/replace} instead

This doesn't work for the $CHROOT_VARIABLES which includes
variables with strings like "/", so we can't rely on the
${variable//search/replace} approach. Quoting from
https://github.com/koalaman/shellcheck/wiki/SC2001:

| Utilizing some of the more complex capabilities of sed is
| required occasionally and it is safe to ignore SC2001.

Quoting from https://lists.debian.org/debian-devel-announce/2019/07/msg00004.html:

| over the last years we had people getting confused over <suite>-updates
| (recommended updates) and <suite>/updates (security updates).  Starting
| with Debian 11 "bullseye" we have therefore renamed the suite including
| the security updates to <suite>-security.
|
| An entry in sources.list should look like
|
|   deb http://security.debian.org/debian-security bullseye-security main
|
| For previous releases the name will not change.

Debian/buster is version "10.0".

When grml-debootstrap is invoked with `--debopt --variant=minbase`, our
handling of /etc/network/interfaces fails, as the directory
/etc/network/ doesn't necessarily exist yet in the essential/minbase
variants (unless you explicitly install ifupdown/netbase).

Closes: #930468

E.g. virtio drivers might have the properties ID_NET_NAME_PATH=enp0s18
and ID_NET_NAME_SLOT=ens18. If we check only for ID_NET_NAME_PATH, then
we end up with a network configuration for enp0s18, while the actual
network interface name will be ens18.

So instead look at all present network devices, iterate over them
(ignoring the virtual interfaces like bridges + vboxnet) and
check for present ID_NET_NAME_* settings, using the following precedence
(as defined in link_config_apply() of systemd/src/udev/net/link-config.c):

* ID_NET_NAME_FROM_DATABASE
* ID_NET_NAME_ONBOARD
* ID_NET_NAME_SLOT
* ID_NET_NAME_PATH
* ID_NET_NAME_MAC

Closes: #929810

If a user requested the --sshcopyid option then it's supposed
to be run successfully, otherwise it might leave an inaccessible
system, so exit with error if execution of `ssh-add -L` fails.

While at it:

* simplify code (no need for $RC)
* don't fail if directory /root/.ssh exists already
* append keys to possibly existing /root/.ssh/authorized_keys
  file, instead of overwriting it

Use locally available public keys to authorise root login on the target
system.

Similar to ssh-copy-id(1) (without the -i option) it checks if `ssh-add
-L` provides any output, and if so those keys are appended to
`/root/.ssh/authorized_keys`.

Closes: grml/grml-debootstrap#128

Predictable Network Interface Names were missing the default
/etc/network/interfaces file when using the --defaultinterfaces option.

Added a Networking-Section in the man page to clarify how the network is
configured in the target system and fixed the documentation for the
options --vmfile and --vm. Both options automatically enable the
--defaultinterface option and not the --nointerfaces option.

Closes: grml/grml-debootstrap#126

Closes: https://github.com/grml/grml-debootstrap/pull/122
Thanks: Patrick Schleizer

There's no reason to say "use /dev/sda1 as system partition"
all the time, when we already mention that we "Install Debian on /dev/sda1".

Provide usage examples for mmdebstrap and LVM devices.

The --iso option also accepts mountpoint without any file: syntax,
which is more human-friendly.

Spotted by lintian (see tag override_dh_auto_test-does-not-check-DEB_BUILD_OPTIONS)

The --mntpoint option is quite unimportant, whereas --grub, --password
and --bootappend all require an argument. Instead the --hostname is
quite important. Adjust accordingly.

Add related debian/post* scripts for proper dpkg-maintscript-helper
rm_conffile support.

Since Zsh version v4.3.12-1 in Debian completions should
go to /usr/share/zsh/vendor-completions and are supported
there out-of-the-box.

The only known incompatibility so far is the --iso ... option.
Therefore abort grml-debootstrap if DEBOOTSTRAP=mmdebstrap
and --iso ... option are used at the same time.

Closes: https://github.com/grml/grml-debootstrap/issues/120

Installation takes ages with lvm 2.03.02-1 if /run/udev isn't
available in the target system, causing messages like:

| WARNING: Device /dev/[...] not initialized in udev database even after waiting 10000000 microseconds.

Especially when grub package gets installed (which invokes
vgs and related tools) this takes forever and looks like a
failing installation.

This /run/udev bind-mount is a workaround required for Debian/buster's
lvm2, at least until #918590 is resolved.

No further changes needed

The "bullseye" release doesn't exist on the Debian repositories
as such yet, though it's supposed to appear at least once buster
(AKA Debian v10.0) is stable.

On 2019-02-12 there's the soft-freeze starting for the Buster release.
Accordingly default to buster as new default within grml-debootstrap.

Closes: grml/grml-debootstrap#117

In September 2018 the backports service was migrated from backports.org
to an official service within the Debian project and repositories (see
https://www.debian.org/News/2010/20100905).

This means http://backports.debian.org/debian-backports no longer exists
as such. Accordingly update the URL to point to the Debian repositories
(using ${RELEASE}-backports).

VirtualBox 5.2.22 is available in current stretch-backports, testing
(buster) and unstable. The URL for VBoxGuestAdditions_5.2.1-118918.iso
no longer works (results in 404).