Use nearest-zone matching for source FTN address when replying to netmail

Rather than only matching exact zone numbers as a final "nearest" address
search, use a "nearest zone" matching algorithm ("nearest" being the most
mathematically near). This way, replying to netmail from a zone 2 address
would always match a zone 1 source address, regardless of how many othernet
addresses are configured and which order they are configured in SCFG.

Fixes issue #447 reported by Charles Blackburn (FBOBBS)