utmp: support non-rootfs configuration

utmp: support non-rootfs configuration

Having a rootfs is not a necessary condition for monitoring utmp, since
/var or /var/run can just be remounted inside the container instead. We
should rely on the other two conditions already in place to decide
whether to monitor the utmp file:

 - the container was started with 'lxc-start', which indicates that it
   has a real init process and is expected to write to a utmp file

 - support for CAP_SYS_BOOT was not found in the kernel, which would
   otherwise supersede utmp monitoring

Signed-off-by: David Ward <david.ward@ll.mit.edu>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>