logind: don't apply RemoveIPC= to system users

logind: don't apply RemoveIPC= to system users

We shouldn't destroy IPC objects of system users on logout.

http://lists.freedesktop.org/archives/systemd-devel/2014-April/018373.html

This introduces SYSTEM_UID_MAX defined to the maximum UID of system
users. This value is determined compile-time, either as configure switch
or from /etc/login.defs. (We don't read that file at runtime, since this
is really a choice for a system builder, not the end user.)

While we are at it we then also update journald to use SYSTEM_UID_MAX
when we decide whether to split out log data for a specific client.