Also update BUILD.txt

This brings down the time needed to enumerate my group database
from 2.4 seconds to 0.15 seconds.

In sssd only local is a native mpg domain, and it is forced.
All other providers will have to unroll mpg users into a user/group pair of
entries in the db. This allows the provider to automatically establish if
the remote server provides mpg users w/o possibily conflicting manual
configurations on the client trying to force an mpg behavior where none
is provided.

This patch reduces the time needed to enumerate groups of a midsized
domain from 12 seconds to 4.4
Optimizes enumerations by doing only 2 ldb searches and some ordering
instead of a number of searches proportional to the number of groups

First pass to remove the legacy option and make it just a property of the
provider

This is part of a set of patches to rewrite sysdb to a hopefully better
API, that will also let use use tevent_req async style calls to manipulate
our cache.

This sysdb_req has always really been a transaction handle and not
a request.

This is part of a set of patches to rewrite transaction support in sysdb to a
hopefully better API, that will also let use use tevent_req async style to
manipulate our cache.

Makes LOCAL a normal backend removing some special handling.
Fix/Add id range filtering and name filtering
Filters uid=0 and gid=0 in the proxy backend as 0 is invalid within
sysdb and was causing getxxent calls to fail completely.
Fix nss_ncache_check_xxx calls to avoid dirtying the 'ret' variable and
causing some unwanted failures.
Change sysdb to always return the uid number when searching member entries so
that id range filtering can be perfomed also in group searhes (does not work
with legacy backends)

Turn user entries to Magic Private Groups when groups are quesried.

Fix infinite loop within initgr functions.
Fix min length check copy&paste error, was filtering valid groups if the name
was short enough and the group had no members.