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.