Fix a couple of issues with external mail processors:

- Since rev 1.556, mailproc_to_match[] would only ever contain one mail
  processor (index) value, even if the RCPT TO address matched multiple
  mail processor "To" keys in mailproc.ini, so only the first-matching
  mail processor (with a "To" key) would be executed for the msg.
- mailproc_match, meaning the destination was a non-passthrough
  external mail processor, in which case the destination (RCPT TO) address
  may *not* match a valid local user account, would be set for *passthru*
  mail processors potentially resulting in the following error (and an
  undelivered mail message): !SMTP can't deliver mail to user #0
  only set mailproc_match to a valid mail proc index if the recipient matches a
  non-passthru mail processor.