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.