Use $BACKEND instead of BDB2.  Fallback to LDBM if $BACKEND is not BDB2.

Just use compiled in defaults.

Integration into ./configure ( --enable-bdb2 ).
Adaptation of the test-suite ( 'make' -> ldbm; 'make bdb2-local' -> bdb2 ).
Minor fixes in slapd/libldbm.

ldap commands.

signals aimed at the process.

BEWARE: the backend will be compiled.
        the backend will NOT be invoked, yet.
        the backend CANNOT be invoked, yet, because it is NOT yet
            integrated into the new initialization/startup environment
            of the slapd server.

  First version with waiting on int to become 0; should be implemented
  via conditional variable...
Fix of typos.
First anchor for new bdb2 backend.

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.

	Kerberos detection of des_debug
	detection of resolv.h and arpa/nameserv.h
Updated affected codes.

check for alias problem
check for alias != entry
check for null entry in subsequent switch statement

okay to read and write LDBM specific fields (state, refcnt,
LRU.  The id field, though is read-only once set.
cache_find_entry_dn2id(), hence, does not require any entry locks.
cache_find_entry_id() must do a entry_rdwr_trylock() and back
off if busy.
Add new rdwr lock code with trylock() functionality.
Implement entry_rdwr_trylock().

frontend-backend interface.
Includes a number changes to share common routines between
database api types.

new frontend-backend interface.

frontend->backend interface.

frontend->backend interface.

LDAP_ALIAS_PROBLEM is now only used if the alias itself is mangled.

fixed deadlocks