Backend documentation patch, version 1

================
Most of this text is taken from OpenLDAP.  The work of rewriting it
to manual pages is done by by Hallvard B. Furuseth and placed into
the public domain.  This software is not subject to any license of
the University of Oslo.
================

Hallvard B. Furuseth <h.b.furuseth@usit.uio.no>, April 2002.

			================
Written by Hallvard B. Furuseth and placed into the public domain.
This software is not subject to any license of the University of Oslo.
			================

maildap could address buf[-1] if len was < 2.

REWRITE_SUBMATCH_ESCAPE is '%', not '\'.

librewrite and saslautz could walk past the end of a string which
ended with an escape character.

Hallvard B. Furuseth <h.b.furuseth@usit.uio.no>, April 2002.

ITS#1677 - cast away const warnings
ITS#1678 - unsigned char args to ctype funcs
ITS#1682 - don't redefine ldap_debug
ITS#1683 - uninitialized vars
ITS#1703 - ldo_debug initialization
ITS#1705 - unsigned testing
ITS#1706 - socklen_t args
ITS#1719 - back-tcl update (other cleanups/fixes/improvements; yet untested)
ITS#1724 - integerNormalize/integerFilter/integerIndexer bugs
ITS#1725 - libdes not required

Implement back-null (/dev/null style backend)
Cleanup some misc warnings ("%lu" format, unused/uninitialized vars,
        ambiguous operator precedence)

Kurt, please regenerate configure

	s/-L.../libraries -lldap/.../libraries/libldap/libldap.la/

Misc cleanup / lint removal

  - librewrite, for string rewriting; it may be used in back-ldap
    by configuring with '--enable-rewrite'. It must be used in
    back-meta. There's a text file, 'libraries/librewrite/RATIONALE',
    that explains the usage and the features. More comprehensive
    documentation will follow.
  - enhancements of back-ldap (ITS#989,ITS#998,ITS#1002,ITS#1054 and ITS#1137)
    including dn rewriting, a fix to group acl matching and so
  - back-meta: a new backend that proxies a set of remote servers
    by spawning queries. It uses portions of back-ldap and the rewrite
    capabilities of librewrite. It can be compiled by configuring
    with `--enable-ldap --enable-rewrite --enable-meta'.
    There's a text file, 'servers/slapd/back-meta/Documentation', that
    describes the main features and config statements.

Note: someone (Kurt?) should run 'autoconf' and commit 'configure' as
my autoconf version must be different: my configures contain a number
of differences and I didn't feel comfortable in adding them :)