netcmd: domain backup offline command

netcmd: domain backup offline command

Unlike the existing 'domain backup online' command, this command allows an
admin to back up a local samba installation using the filesystem and the
tdbbackup tool instead of using remote protocols.  It replaces samba_backup
as that tool does not handle sam.ldb and secrets.ldb correctly.  Those two
databases need to have transactions started on them before their downstream
ldb and tdb files are backed up.

Signed-off-by: Aaron Haslett <aaronhaslett@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Gary Lockyer <gary@catalyst.net.nz>