Split tags: Introduce $mktag in push_mktag

push_mktag is going to want to become push_mktags, and make perhaps
two tags.

For now, break the actual tag-making out into a subref.  Rename the
arguments to push_mktag to ensure we caught all the references in
$mktag.

push_mktag now wants to return an array; make sure that the call sites
all give it array context.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>