Unify gsu_dir handling.

The three locations which include the common part of gsu need to know
the gsu installation directory. Currently we rely on the application
to set $gsu_dir. This is unnecessary since gsu can tell where it is
installed by examining ${BASH_SOURCE[0]}.

Also the fallback to $HOME/.gsu is not required. This commit unifies
all three locations and also calls _gsu_setup as early as possible:
when the gsu module is sourced.