configure: warn on invalid ca bundle or path

- Warn if --with-ca-bundle file does not exist.

- Warn if --with-ca-path directory does not contain certificates.

- Improve help messages for both.

Example configure output:

  ca cert bundle:   /some/file   (warning: certs not found)
  ca cert path:     /some/dir   (warning: certs not found)

Bug: https://github.com/curl/curl/issues/404
Reported-by: Jeffrey Walton