Allow for specifying path to netrc as an option

[luizribeiro]

From commit message:

This allows users to give the location to the netrc file, which often contains secrets for authentication into an FTP server.

Typically, this file lives under $HOME/.netrc. However, in certain scenarios it is desirable to keep this file somewhere other than the user's directory.

This is basically the same patch from https://sourceforge.net/p/curlftpfs/discussion/542749/thread/3becfed6/ which was never merged upstream.

While at it, I also removed an unused function that was breaking compilation when warnings are enforcing.