(almost) everything for bug#375 - create export_svc table - part_svc to

part_export is now properly many-to-many, not one-to-many

still need to finish service editing (choosing exports) in
httemplate/edit/part_svc.cgi and httemplate/edti/process/part_svc.cgi

and do somethinga about that manual $svcdb = 'svc_acct' in
httemplate/edit/part_export.cgi (do part_export records need a svcdb?
probably not... should be able to just pass an svcdb on creation of new
exports, move the big %exports hash into part_export.pm and allow
httemplate/edit/part_svc.cgi to query it for exports that can apply to a given
svcdb....