Properly parse multi-line FTP responses

[nnposter]

The current behavior of ftp.read_reply() is to treat any response line that starts with three decimal digits and a space as the end of a multi-line response. However, this is not the correct behavior according to RFC 959, section 4.2:

Thus the format for multi-line replies is that the first line will begin with the exact required reply code, followed immediately by a Hyphen, "-" (also known as Minus), followed by text. The last line will begin with the same code, followed immediately by Space , optionally some text, and the Telnet end-of-line code.
...
The user-process then simply needs to search for the second occurrence of the same reply code, followed by (Space), at the beginning of a line, and ignore all intermediary lines. If an intermediary line begins with a 3-digit number, the Server must pad the front to avoid confusion.

This patch updates ftp.read_reply() to comply with the RFC. It will be merged in after February 1 unless concerns are raised.