Add a magic number to debug_locks to better catch lock-coding errors.

Add a magic number to debug_locks to better catch lock-coding errors.

Original description by Dave Hart:

 [This patch contains] the addition of a signature field to debug_lock
 initialized by the alloc routine and verified by the rest, to catch
 invalid lock pointers sooner. That helped me track down a nasty
 problem I had before adding the time.h include to
 libevent-internal.h, where different .c files had different ideas of
 whether event_base had a tod_tv_cache member depending on which
 included time.h before libevent-internal.h.