netrc: handle longer username and password

libcurl truncates usernames and passwords it reads from .netrc to
LOGINSIZE and PASSWORDSIZE (64) characters without any indication to
the user, to ensure the values returned from Curl_parsenetrc fit in a
caller-provided buffer.

Fix the interface by passing back dynamically allocated buffers
allocated to fit the user's input.  The parser still relies on a
256-character buffer to read each line, though.

So now you can include an ~246-character password in your .netrc,
instead of the previous limit of 63 characters.

Reported-by: Colby Ranger