Minor factoring in C4/Biblio.pm, plus change to export the per-tag

Minor factoring in C4/Biblio.pm, plus change to export the per-tag
'mandatory' property to a per-subfield 'tag_mandatory' template parameter,
so that addbiblio.tmpl can distinguish between mandatory subfields in a
mandatory tag and mandatory subfields in an optional tag

Not-minor factoring in acqui.simple/addbiblio.pl to make the if-else blocks
smaller, and to add some POD; need further testing for this

Added function to check if a MARC subfield name is "koha-internal" (instead
of checking it for 'lib' and 'tag' everywhere); temporarily added to Koha.pm

Use above function in acqui.simple/additem.pl and search.marc/search.pl