netrc caching and better searching.

netrc caching and better searching.

* auth-source.el: Don't load netrc.el.
(auth-source-debug): Add 'trivia option for extra output.
(auth-source-do-trivia): Use it.
(auth-source-search): Simplify logic to use
`auth-source-search-backends'.  Use `auth-source-do-trivia' where
appropriate.  Don't keep a running count at this level.  Layer :create
and :delete options appropriately on the first and second passes.
Don't track the backend with the search results.
(auth-source-search-backends): New function to search a list of
backends for a processed spec.
(auth-source-netrc-parse): Cache all netrc files, making
auth-source-netrc-cache an alist keyed by the file name and using the
file mtime as the caching criterion.  Keep the obfuscated data secret
with a lexical bind.
(auth-source-netrc-search): Don't calculate the length of the results
unnecessarily.