parse_remote_port: fix ;type= URL suffix over HTTP proxy

Test 563 is enabled now and verifies that the combo FTP type=A URL,
CURLOPT_PORT set and proxy work fine. As a bonus I managed to remove the
somewhat odd FTP check in parse_remote_port() and instead converted it
to a better and more generic 'slash_removed' struct field. Checking the
->protocol field isn't right since when an FTP:// URL is sent over a
HTTP proxy, the protocol is HTTP but the URL was handled by the FTP code
and thus slash_removed is set TRUE for this case.