curl_multi_fdset: correct fdset with FTP PORT use

After a PORT has been issued, and the multi handle would switch to the
CURLM_STATE_DO_MORE state (which is unique for FTP), libcurl would
return the wrong fdset to wait for when curl_multi_fdset() is
called. The code would blindly assume that it was waiting for a connect
of the second connection, while that isn't true immediately after the
PORT command.

Also, the function multi.c:domore_getsock() was highly FTP-centric and
therefore ugly to keep in protocol-agnostic code. I solved this problem
by introducing a new function pointer in the Curl_handler struct called
domore_getsock() which is only called during the DOMORE state for
protocols that set that pointer.

The new ftp.c:ftp_domore_getsock() function now returns fdset info about
the control connection's command/response handling while such a state is
in use, and goes over to waiting for a writable second connection first
once the commands are done.

The original problem could be seen by running test 525 and checking the
time stamps in the FTP server log. I can verify that this fix at least
fixes this problem.

Bug: http://curl.haxx.se/mail/lib-2011-10/0250.html
Reported by: Gokhan Sengun