curl: don't set FTP options for FTP-disabled builds

... since libcurl has started to be totally unaware of options for
disabled protocols they now return error.

Bug: https://github.com/curl/curl/commit/c9c5304dd4747cbe75d2f24be85920d572fcb5b8#commitcomment-33533937

Reported-by: Marcel Raad
Closes #3886