serialize multiple threads doing lxcapi_start()

The problem is that the fd table is shared between threads and if a thread
forks() while another thread has an open fd to the monitor, the duped fd
in the fork()ed child will not get closed, thus causing monitord to stay
around since it thinks it still has a client. This only happened when
calling lxcapi_start() in the daemonized case since that is the only time
we try to get the status from the monitor.

Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>