smtp: Simplified the next RCPT TO logic
DOCS: Updated manpage following recent SMTP modifications

* Added SMTP section to --request
* Expanded --mail-rcpt to describe the usage when using the verify and
  expand commands.
DOCS: Updated curl_easy_setopt.3 following recent SMTP changes

* Added information about the verify and expand commands to
  CURLOPT_MAIL_RCPT.
* Reworked CURLOPT_CUSTOMREQUEST section, adding information about IMAP
  and SMTP custom commands.