libctdb: use magic to detect free/invalid locks

libctdb: use magic to detect free/invalid locks

Rather than using a binary, we use a magic value for locking.  We also
split out the "dont have the lock yet" from the "do have the lock"
paths for clarity and extra checking.

This should detect a superset of the previous case, even if they free
(and reuse) the lock memory.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>