MyDNS

when admin uses foreign keys to control RR types
instead of enumerated type in "type" column, the
admin.php is unable to detect what RR types are3 available.

This patch adds foreign keys support to admin.php.
admin just needs to set up table used for RR types.

the foreign key column needs to have the name "type"
(it's hardcodedm but other column names are hardcoded
too, so I don't found this to be an issue).