The fails in the previous test showed that we need the first three
changes here. Some final polishing in points 4 to 6.
 Sub merge: Refine the condition for initializing $tags_new.
A postponed 'modify'-merge (A to B) makes that $authtypefrom is not
defined when running merge later. When crossing the type boundary, we
need a new field too.
 Sub merge: Add condition for an empty @record_to array.
This indicates also that a field should be removed, since we should
otherwise only add a $9 subfield.
 Sub merge: Adjust initializing @record_from.
This change is tested by verifying a cleared subfield in the test.
 DelAuthority: Adding a skipmerge parameter to allow the call from
authorities/merge.pl to skip an unneeded merge.
This also prevents that the 'delete-merge' would precede the
'modify-merge' under a hypothetical race condition.
 DelAuthority: There is actually no need to call GetAuthority.
The subfields of the old record are not relevant in this case.
 Added a few POD lines to merge.
 Removed a trailing space in a comment line in merge.
 Run t/db_dependent/Authorities/Merge.t.
The last subtest should no longer fail now.
 See test plan of next patch.