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.

Don't actually perform the _last_ dimming step, so it's more obvious that the screen can still be unlocked by just moving the mouse.

This ensures that the screen going black always indicates that it's safe
to leave the computer alone.

This improves handling of the "en_US" keymap, which before this change
showed up without a name, and with this change shows up as
"pc+latin1+inet(evdev)". Well, better than nothing.

Internal reference: b/74086590.

If selected saver is "one" but the saver index can't be found, assume
"random-same" again like in the past.

This implements the distinction between "Random Screen Saver" (different
per screen) and "Same Random Saver" (same saver on all screens) in
xscreensaver-demo's settings.

Visibly here means that the cursor position is not updated, to make
clear to the user that these keys do nothing.

Related to #25.

Support Ctrl-U to clear the password entry line, and document behavior of some other screen lockers regarding this.

Leaving open how to handle Ctrl-A yet though.

Related to #25.