x11/xfce4-terminal: Enable utmp/wtmp support
Unconditionally enable utempter support.
This change adds no extra dependencies, since a compatible
implementation is already present in FreeBSD base since 9.0, and
adds back functionality that was lost in xfce4-terminal after version
0.8.0.
The functionality can be controlled via the configuration UI.
Requested/Suggested by: Daniel O'Connor <darius@dons.net.au>