whoops :/

Add direct compatibility to XScreenSaver hacks by passing -root as first argument to all saver children.

This now works:

XSECURELOCK_SAVER=/usr/lib/xscreensaver/discoball xsecurelock.

All our own saver children ignore argv, so this is a compatible change.

- It's right next to Ctrl-Alt-L on most standard keyboard layouts
  (QWERTY, QWERTZ, AZERTY) so easy to use as a double key combo
  Ctrl-Alt-LO (rather than Ctrl-Alt-LL) to lock and switch.

- It prevents accidental double-taps on L from invoking the user switch
  (annoys some users).

- auth_ prefix now applies to the basename, not the entire path.
- Absolute paths are allowed too as alternatives to paths within the
  helpers path.
- Savers can now be any binary paths, as long as they do not start with
  auth_ (i.e. now auth_ is forbidden rather than only saver_ allowed).

Fixes #35.

Might be useful with large fonts and/or 4k screens.

XSECURELOCK_AUTH_TIMEOUT (default: 300): number of seconds to wait for
user input on an auth prompt. This actually matches the previous
hardcoded setting, but is configurable now.

XSECURELOCK_BURNIN_MITIGATION (default: 16): do not always display the
auth prompt at the same place, but allow it to be off center by up to
the given amount of pixels. Helps when the auth screen times out but
reappears for some reason (e.g. a vibrating desk surface moving the
mouse).

XSECURELOCK_BURNIN_MITIGATION_DYNAMIC (default: off): allow the auth
prompt move during a single auth session. Many will find this annoying,
but it really should fix (by blurring out) any burn-in effects.

The subprocess code calls pipe(), dups the pipe fd to 0 and then closes
the pipe. However, if fd 0 is unconnected, pipe() may generate fd 0 and
wreak havoc.

The updated code handles this case:

- If the write end is already 0, no dup2 and close needed for it.
- If the read end is 0, the dup2 of the write end onto 0 already closed it.

- Fix error checking (previous code failed to report errors).
- Don't F_SETFD if F_GETFD failed.
- Read the environment variable only once.

Also, reducing the border (amount of extra pixels around the draw area
to clear) from 16 to 4 pixels as this should be safe now.

- Include final space for blinking cursor in text width (so the **** no
  longer move).
- Remove tw_cursor, replace by a static margin.

Still don't know why we need the margin in the first place (i.e. why
fonts sometimes draw outside their bbox).

Also, add dependency on xset, and suggestion for mplayer and mpv.

Reduces log spam, changes nothing else.

- Enable Xfixes and Xsync support.
- Specify all configure with/without flags to disable autodetection.
- Update build dependencies.

Fixes #29.

This makes the default mode of operation work when compiling just
against Xsync, but not Xss.

Related to #30.

Fixes #30.