auth-source library supports Tramp, Gnus, smtpmail, URL auth

[Ted Zlatanov]

I think the auth-source library is ready for normal usage.  It supports
Tramp (with a patch I just comitted, so it will get to the Emacs CVS
when Michael Albinus synchronizes), Gnus (nnimap, nntp, and mail source
(IMAP, POP) passwords), SMTP (smtpmail), and URL authentication.  Thus
the users can put all their Emacs passwords in one place and they will
just work.  Right now by default that's ~/.authinfo.gpg, which works
nicely with epa-file-mode.

There are some issues still left:

It requires netrc.el and doesn't cache the parse results (the authinfo
file will be reopened each time, which is slow with epa-file-mode[1]).
OTOH this ensures that whatever you put in the authinfo file will be
used immediately, and considering the low frequency of authentication
calls I think that's not a bad compromise.

It should move out of Gnus since it's used in many places outside.  It
uses gnus-message but that's easy to fix.

It should be documented externally (in its own manual or in the Emacs
manual, but the Gnus and Tramp manuals will need to link to the
information regardless).

Only authinfo/netrc-style files are currently supported, which has so
far been sufficient, but maybe we need others.

I'd appreciate any comments or suggestions.

Ted

[1] I am using epa-file-mode, and compared to my old encrypt.el code
it's much slower to decrypt a file.  Can this be optimized or is it
necessary for security?  I don't know the internals so take this as a
naive question.