ldb/attrib_handler casefold: simplify space dropping

ldb/attrib_handler casefold: simplify space dropping

As seen in CVE-2021-20277, ldb_handler_fold() has been making mistakes
when collapsing spaces down to a single space.

This patch fixes the way it handles internal spaces (CVE-2021-20277
was about leading spaces), and involves a rewrite of the parsing loop.

The bug has a detailed description of the problem.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14656

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Wed Apr  7 03:16:39 UTC 2021 on sn-devel-184