ftp: fix interrupt callback misuse

FTP protocol used interrupt callback to simulate nonblock
operation which is a misuse of this callback.

This commit make FTP protocol fully blocking and removes
invalid usage of interrutp callback

Also adds support for multiline responses delimited with dashes
ftp: remove unused headers
ftp: comments
ftp: warning about pure-ftp server used as and output
Merge https://github.com/lukaszmluki/ffmpeg

* https://github.com/lukaszmluki/ffmpeg:
  ftp: warning about pure-ftp server used as and output
  ftp: comments
  ftp: remove unused headers
  ftp: fix interrupt callback misuse

Merged-by: Michael Niedermayer <michaelni@gmx.at>