Address sysops sending emails to net-coordinators w/invalid reply-to address

Issue raised by Dumas Walker (CAPCITY2) on DOVE-Net: apparently some new
sysops run this script before their configured hostname (e.g.
mybbs.synchro.net) is valid or their mail server has been tested (can
successfully receive Internet e-mail).

This attempts to address this concern by:
1. displaying a warning that it's important that the address given is valid
2. attempt to validate that the host portion of the provided address is valid
   (has a DNS address record or MX record)

Uses dns.js for MX-record lookup - thanks Deuce!

This script does not validate that the email host can actually receive mail
or that the name portion of the mail address is valid: that would require an
outbound connection to the host's TCP port 25, which may be blocked by ISPs
(if remote) or if its a local server (e.g. the same machine as the BBS), that
wouldn't confirm that its reachable by Internet hosts. And not all mail
servers support the necessary SMTP commands to validate recipient addresses.