SF bug #1168983:  ftplib.py string index out of range
SF bug #1168983:  ftplib.py string index out of range

* resp[:1] in '123' # after Py2.2, this allowed blank responses to pass.
* replace <> with !=
* provide a usage message for empty command line calls

Backport candidate.

sk-/python2.7-type-annotator,sk-/python2.7-type-annotator,sk-/python2.7-type-annotator