LDBM/back-ldbm locking reworked!

	back-ldbm was using a cache specific lock to protect non-reentrant
	database routines from being reenterred.  This is inadequate.
	Also, reentrant database systems calls were serialized unnecessarily.

	Non-reentrant database calls must have a big_mutex.  Implemented
	this within -lldbm itself.  library requires ldbm_initialize()
	be called before any other ldbm call to initialize the big_mutex
	and to do any other db specific initialization (ie: such as
	required for DB2).

The dbc_mutex, dbc_cv, & dbc_readers fileds of dbcache are history.
The "reentrant_database" (REENTRANT_DATABASE) define is also axed.