Set an rpath entry to WITH_SYSTEMDIR/lib

This automagically sets an rpath to WITH_SYSTEMDIR/lib, when
WITH_SYSTEMWIDE is specified. If no WITH_SYSTEMDIR is given,
the default path /usr/share/games/quake2/lib is used. As for
the normal rpath, for now only FreeBSD and Linux are supported.
This was requested by caedes.
Wrap too long line
Add an signal handler for SIGINT and SIGTERM

This allows the user to shut the client down by pressing ctrl-c in it's
terminal or by sendig SIGTERM.
Update CHANGELOG for 5.21
Bump version number to 5.21