units: remove "AllowUser=root own", the bus owner can always own names
fix off by one error in array index assertion

Since the index is already post-incremented when the array is appended
to, this assertion can be wrongly reached when the array is at capacity
(with the NULL terminator). The bug is reproducible on shutdown with
the following settings in /etc/systemd/system.conf:

  LogTarget=journal-or-kmsg
  LogColor=yes
  LogLocation=yes

Reported by Thermi on IRC.