We were doing the check for whether we need to watch utmp from a
thread cloned from that which will actually do the utmp watching.
As a result, the utmp file was always being watched, even if it
didn't need to be.

Move the check to the parent thread.

Bug-Ubuntu: https://bugs.launchpad.net/ubuntu/+source/lxc/+bug/948623Signed-off-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>