ftp: fix Curl_ftpsendf()

... it no longer takes printf() arguments since it was only really taken
advantage by one user and it was not written and used in a safe
way. Thus the 'f' is removed from the function name and the proto is
changed.

Although the current code wouldn't end up in badness, it was a risk that
future changes could end up springf()ing too large data or passing in a
format string inadvertently.