ftpserver.pl: Fixed variable error introduced in 7fd84b14d219b1

Global symbol "$mailbox" requires explicit package name
ftpserver.pl: Fixed missing comma from 7fd84b14d219b1
libcurl.3: for multi interface connections are held in the multi handle

... and a few more cleanups/clarifications
libcurl: New options to bind DNS to local interfaces or IP addresses
curl: Add support for various DNS binding options.

(Passed on to c-ares.)

Allows something like this:

curl --dns-interface sta8 --dns-ipv4-addr 8.8.1.111 --interface sta8 \
--localaddr 8.8.1.111 --dns-servers 8.8.8.1 www.google.com

Signed-off-by: Ben Greear <greearb@candelatech.com>
curl_easy_setopt.3: mention RTMP URL quirks

URL: http://curl.haxx.se/bug/view.cgi?id=1278
Reported-by: Gorilla Maguila
curl: Fixed usage of DNS options when not using c-ares resolver

Commit 32352ed6adddcb introduced various DNS options, however, these
would cause curl to exit with CURLE_NOT_BUILT_IN when c-ares wasn't
being used as the backend resolver even if the options weren't set
by the user.

Additionally corrected some minor coding style errors from the same
commit.
curl.1: detail how short/long options work

URL: http://curl.haxx.se/bug/view.cgi?id=1279
Suggested-by: Jerry Krinock
tests: Updated descriptions to be more meaningful
imap: Fixed response check for NOOP command
ftpserver.pl: Added support for IMAP NOOP command
tests: Updated descriptions to be more meaningful
ftpserver.pl: Added capability prerequisite for extended POP3 commands
ftpserver.pl: Corrected invalid argument check in POP3 TOP handler

...which was accidentally introduced in commit 4d6ef6297ae9b6.
ftpserver.pl: Moved POP3 USER and PASS handlers into own functions
ftpserver.pl: Moved IMAP LOGIN handler into own function
ftpserver.pl: Corrected invalid user details check

...in both the IMAP LOGIN and POP3 PASS handlers introduced in commit
187ac693744949 and 84ad1569e5fc93 respectively.
ftpserver.pl: Added mailbox check to IMAP SELECT handler
ftpserver.pl: Added support for SMTP HELO command

...and updated test902 as explicit HELO response is no longer required.
ftpserver.pl: Moved SMTP EHLO and QUIT handlers in own functions