exporttype2svcdb vs. export_info

[Mark Wells]

The FS::part_export::exporttype2svcdb function breaks on exporttypes that
handle more than one svcdb.

It's also strange in that it's only used in one place (edit/part_svc.cgi),
to produce a list of all exports matching a particular svcdb.  Isn't
export_info better suited for that?  (See my latest patch.)  Aside from
not imposing a one-svcdb-per-exporttype restriction, it follows the data
structure better (svcdb is the top level in the hierarchy, then
exporttype, then options).

If we must keep exporttype2svcdb, I suggest that it return a list of _all_
matching svcdbs for the given exporttype.