dcesrv_core: wrap gensec_*() calls in [un]become_root() calls

dcesrv_core: wrap gensec_*() calls in [un]become_root() calls

This is important for the source3/rpc_server code as it might
be called embedded in smbd and may not run as root with access
to our private tdb/ldb files.

Note this is only really needed for 4.15 and older, as
we no longer run the rpc_server embedded in smbd,
but we better be consistent for now.

This should be able to fix the problem the printing no longer works
on Windows 7 with 2021-10 monthly rollup patch (KB5006743).

Windows uses NTLMSSP with privacy at the DCERPC layer on top
of NCACN_NP (smb).

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14867

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>