utmpupdate: Add a program for updating /var/run/utmp

To keep track of ongoing terminal sessions, we now have a sort-of
traditional /var/run/utmp file, like other Unix systems.
Unlike other Unix systems however, ours is of course JSON. :^)

The /bin/utmpupdate program is used to update the file, which is
not writable by regular user accounts. This helper program is
set-GID "utmp".

Base/etc/fstab

@@ -7,6 +7,7 @@
 /etc	/etc	bind	bind,nodev,nosuid
 /home	/home	bind	bind,nodev,nosuid
 /root	/root	bind	bind,nodev,nosuid
+/var	/var	bind	bind,nodev,nosuid
 
 none	/proc	proc	nosuid
 none	/dev/pts	devpts	noexec,nosuid,ro