Add DHCP setting for Predictable Network Interface Names

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