netrc.el now supports encoded files

[Ted Zlatanov <tzz@lifelogs.com>]

I used the very timely password.el to add password support to
netrc.el, and it can now decode symmetric ciphers with OpenSSL.  The
OpenSSL binary is required, so it would be nice if someone implemented
a built-in cipher.  A helper netrc-encode function is also provided.

I have not updated the docs, please test and/or see the code if you
are interested.  It works for me, both without encodind and with it.
I'll update the docs later, but it's likely that the interface will
change before then.

Ted