Finish ftp refactoring. Test suite passing.
* bzrlib/transport/ftp.py:
(FtpTransport.__init__): Simplified.
(FtpTransport._unparse_url): Deleted.
(FtpTransport._abspath): Deleted and reimplemented as a
specialisation of _remote_path. The heart is: we don't support
Unicode paths :-/
(FtpTransport.abspath): Deleted.