locking.c, locking.1: Make the protocol safe if the lockfile is (re)moved.

Get the device/inode pair from the lockfile descriptor after we've
opened it (so this is the file we're actually going to lock).  Once
we've acquired the lock, check that the file /name/ still has the same
device/inode pair.  If nobody is allowed to move or unlink the lockfile
while we've got the lock, then we know (assuming this check passes) that
we've actually locked the right file and not some deleted thing.