ftpserver.pl: Added the ability to specify custom full text replies
ftpserver.pl: Added the ability to send custom full text replies
ftpserver.pl: Fixed runtime warning from commit 7da9c95bcf1fe6

Use of uninitialized value $FTPARG in concatenation (.) or string at
line 3255.
FILE: don't wait due to CURLOPT_MAX_RECV_SPEED_LARGE

The FILE:// code doesn't support this option - and it doesn't make sense
to support it as long as it works as it does since then it'd only block
even longer.

But: setting CURLOPT_MAX_RECV_SPEED_LARGE would make the transfer first
get done and then libcurl would wait until the average speed would get
low enough. This happened because the transfer happens completely in the
DO state for FILE:// but then it would still unconditionally continue in
to the PERFORM state where the speed check is made.

Starting now, the code will skip from DO_DONE to DONE immediately if no
socket is set to be recv()ed or send()ed to.

Bug: http://curl.haxx.se/bug/view.cgi?id=1312
Reported-by: Mohammad AlSaleh
docs: mention CURLOPT_MAX_RECV/SEND_SPEED_LARGE don't work for FILE://
tests: Added IMAP SASL downgrade tests
tests: Added POP3 SASL downgrade tests
tests: Added SMTP SASL downgrade tests
tests: Corrected syntax error from commit 7246255416617a
ftpserver.pl: Updated custom full text REPLY regex

SASL downgrade tests: 833, 835, 879, 881, 935 and 937 would fail as
they contained a minus sign in their authentication mechanism and this
would be missed by the custom reply parser.
test936: Corrected login details from commit 7246255416617a