New Frontend->Backend Interface

New Frontend->Backend Interface
Separates per backend type from per backend database initialization
	and startup.  Also supports per type / per backend shutdown.
New frontend startup/shutdown routines are also provided:
	slap_init() slap_startup() slap_shutdown() slap_destroy()
New frontend->backend startup/shutdown is managed by:
	backend_init() backend_startup() backend_shutdown backend_destroy
backend_init() now calls bi_init() to initial all function pointers
for the backend (excepting bi_init() which is now the only hardcoded
entry point).  New entry points are detailed in slap.h struct
backend_info.  backend_info is a per database type structure.
Besides the new startup/shutdown entry points, the new interface
also supports per backend type configuration options.  One could have:

	backend bdb2	(new Berkeley DB 2 backend)
	bdb2_home	/directory

	database bdb2
	...

	*** This code is fairly experimental ***
	*** Much cleanup and testing is still needed ***

see slap.h for details on struct backend_db and backend_info.