- 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.

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.