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.

Don't ask, it simply looks nicer this way. Feel free to convince me
otherwise.

Now the only differences between a fresh clone + sh autogen.sh and make
dist are:

- autom4te.cache (unnecessary)
- .gitignore (VCS related)

Doing this in a separate commit as it's somewhat controversial. See
comment.

This is achieved by mapping it to Ctrl-U in the pipe channel. Ctrl-U is
unix-line-discard in most input fields.

The default saver stays saver_blank.

Avoids installing a broken saver_xscreensaver.

- Add some missing X*QueryExtension calls.
- Make #ifdef identifiers consistent: HAVE_X*_EXT.
- Make test result variables consistent: have_x*_ext.
- Simplify non-Xkb handling in auth_pam_x11.
- Update linter config.

until_nonidle: also kill the dimming tool after a timeout (same values as the ones dimmer is using).

Should be 100% compatible when using dimmer, and enables use of
dim-screen.sh from the xss-lock package.