Netrc field encryption support.

Netrc field encryption support.

* auth-source.el (auth-source-save-secrets): New variable to control if
secret tokens should be saved encrypted.
(auth-source-netrc-parse, auth-source-netrc-search): Pass the file name
to `auth-source-netrc-normalize'.
(with-auth-source-epa-overrides): Add convenience macro.
(auth-source-epa-make-gpg-token): Convert text to a "gpg:" token.
(auth-source-netrc-normalize): Convert "gpg:" tokens back to text in
the lexical-let closure.
(auth-source-netrc-create): Create "gpg:" tokens according to
`auth-source-save-secrets'.