QWK import/export now correctly handles the unexpected case of RECIPIENTNETTYPE

and RECIPIENTNETADDR fields in messages posted to sub-boards. Apparently
GoldEd+ does this and it triggered a chain of bugs (mostly cosmetic), but bugs
that bugged me, so I'm squashing them. Now messages being export to QWK will
not include the RECIPIENTNETTYPE/ADDR information unless it's netmail.
Ditto for importing. Also, the net_type is more accurately determined (i.e. if
there's no '@' in the address, as there not be any expected for QWK and Fido
net addresses).