s4/dns_server: split out function that does the MAC computation

s4/dns_server: split out function that does the MAC computation

Split out function that does the MAC computation from the TSIG record
creating function. This will later simplify the code when creating error
responsed to TSIG requests with bad MACs where we have to add the TSIG
record with an empty MAC.

No functional behaviour change besides hard coding "gss-tsig" algorithm
name: later when sending a TSIG error response for a TKEY request with a
bad keyname, we won't have a tkey to fetch the algorithm name from.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=11520

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>