common/groups.sh: lower FIRST_SYSTEM_UID to 100

After performing an upgrade, packages were broken on every jessie machine:

ckuehl@avalanche:~$ sudo dpkg --configure -a
Setting up dbus (1.8.10-1) ...
adduser: The user `messagebus' already exists. Exiting.
dpkg: error processing package dbus (--configure):
 subprocess installed post-installation script returned error exit status 1

The messagebus user has uid < 300 on all the jessie desktops (it's >=
300 on the wheezy ones), so it's not seen as a system user, and when the
upgrade tries to do `adduser --system` to ensure the system user exists,
it fails.

This isn't really a jessie bug; we had local system users with uid < 300
on wheezy, too, but never ran in to actual problems because of it.

There's some more context in rt#3096, plus rt#301, rt#299, and rt#61
(provided by Dara).

There are no LDAP users with uid < 500, so this move should be safe and
allows us to fix these broken packages on the jessie machines.