Bug 9988: Few subtle changes for postponed merge

Bug 9988: Few subtle changes for postponed merge

The fails in the previous test showed that we need the first three
changes here. Some final polishing in points 4 to 6.

[1] 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.
[2] 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.
[3] Sub merge: Adjust initializing @record_from.
    This change is tested by verifying a cleared subfield in the test.
[4] 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.
[5] DelAuthority: There is actually no need to call GetAuthority.
    The subfields of the old record are not relevant in this case.
[6] Added a few POD lines to merge.
[7] Removed a trailing space in a comment line in merge.

Test plan:
[1] Run t/db_dependent/Authorities/Merge.t.
    The last subtest should no longer fail now.
[2] See test plan of next patch.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Jacek Ablewicz <abl@biblos.pk.edu.pl>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>