Improve the netrc field encryption (GPG tokens) by calling EPA/EPG functions directly.

Improve the netrc field encryption (GPG tokens) by calling EPA/EPG functions directly.

* auth-source.el: Require EPA and EPG.
(auth-source-passphrase-alist): New variable.
(auth-source-passphrase-callback-function)
(auth-source-token-passphrase-callback-function): Callbacks for the
netrc field encryption (GPG tokens).
(auth-source-epa-extract-gpg-token, auth-source-epa-make-gpg-token):
Symmetric encryption and decryption of the netrc GPG tokens.
(auth-source-netrc-normalize): Use them, simplifying the closure.