ftpserver.pl: Moved cURL SMTP server detection into EHLO command handler

Moved the special SMTP server detection code from the DATA command
handler, which happens further down the operation chain after EHLO,
MAIL and RCPT commands, to the EHLO command as it is the first command
to be generated by a SMTP operation as well as containing the special
"verifiedserver" string from the URL.

This not only makes it easier and quicker to detect but also means that
cURL doesn't need to specify "verifiedserver" as --mail-from and
--mail-rcpt arguments.

More importantly, this also makes the upcoming verification changes to
the RCPT handler easier to implement.