dsdb: make get_parsed_dns_trusted() a common helper function

dsdb: make get_parsed_dns_trusted() a common helper function

We are already using it in two places, and are about to add a third.

The version in repl_meta_data.c did more work in the case that the
parsed_dns can't really be trusted to conform to the expected format;
this is now a wrapper called get_parsed_dns_trusted_fallback().

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>