Fix default prompt and repeated saver query bugs.

Fix default prompt and repeated saver query bugs.

* auth-source.el (auth-source-format-prompt): Always convert the value
to a string to avoid evaluating non-string arguments.
(auth-source-netrc-create): Offer default properly, not as initial
content in `read-string'.
(auth-source-netrc-saver): Use a cache keyed by file name and MD5 hash
of line to determine if we've been run before.  If so, don't run again,
but print a trivial message to indicate the cache was hit instead.