Add compilation option for login completion
Remove X support
config.h: Tweak prompt
Makefile: Switch prefix to /usr
Don't block signals, as it caused issues

Blocking signals doesn't seem to be required (xlsh will be restarted
automatically anyways, just as when using command exit (login doesn't
block them either)) but blocking them can cause trouble, e.g. when
using systemd, it could lead to systemd sending a signal and wait, but
since it was blocked one needed to manually use command exit for xlsh
to end/system to shutdown.

(There was also the issue that blocking SIGINT/SIGHUP was not restoring
them upon login would have them still blocked with the session, leading
to issues thereafter.)
Set environment from PAM
Add .gitignore
Makefile: Install in $prefix/bin not $prefix/sbin