Added configuration parameter --disable-locking for systems that don't

Added configuration parameter --disable-locking for systems that don't
have a complete pthread_mutex_t data type. A complete data type is
required because the mutex is stored in shared memory and a pointer
won't work. FreeBSD and probably others have this problem. I'll
probably end up rewriting pwmd to use multi-threading exclusively so
this won't be a problem anymore. Until then, for systems that have
this limitation, the chances of data file corruption are increased
quite a bit if two processes have the same file open and both try to
write. This also affects the key cache when two processes want to
change the key when saving.