daemon: Fix behavior of read_pidfile() for our own pidfile.

daemon: Fix behavior of read_pidfile() for our own pidfile.

Opening a file descriptor and then closing it always discards any locks
held on the underlying file, even if the file is still open as another file
descriptor.  This meant that calling read_pidfile() on the process's own
pidfile would discard the lock and make other OVS processes think that the
process had died.  This commit fixes the problem.