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".

Meta/build-root-filesystem.sh

@@ -6,6 +6,7 @@ wheel_gid=1
 tty_gid=2
 phys_gid=3
 audio_gid=4
+utmp_gid=5
 window_uid=13
 window_gid=13
 
@@ -51,11 +52,13 @@ chmod 4755 mnt/bin/ping
 chmod 4750 mnt/bin/reboot
 chmod 4750 mnt/bin/shutdown
 chmod 4750 mnt/bin/keymap
+chown 0:$utmp_gid mnt/bin/utmpupdate
+chmod 2755 mnt/bin/utmpupdate
 
 echo "done"
 
 printf "creating initial filesystem structure... "
-for dir in bin etc proc mnt tmp boot mod; do
+for dir in bin etc proc mnt tmp boot mod var/run; do
     mkdir -p mnt/$dir
 done
 chmod 700 mnt/boot
@@ -63,6 +66,12 @@ chmod 700 mnt/mod
 chmod 1777 mnt/tmp
 echo "done"
 
+printf "creating utmp file... "
+touch mnt/var/run/utmp
+chown 0:$utmp_gid mnt/var/run/utmp
+chmod 664 mnt/var/run/utmp
+echo "done"
+
 printf "setting up device nodes... "
 mkdir -p mnt/dev
 mkdir -p mnt/dev/pts