[bug-mailutils] Const craziness.

[Jeff Bailey]

Jordi asked me to make sure that things were sane on 64bit, and I'm going
through and making sure all header files are included and stuff.

I came across this bit of silliness, that doesn't seem right, but I'm
not sure of the best solution:

(mailbox/url.c)
const char *
url_to_string (const url_t url)
{
  if (url == NULL || url->name == NULL)
    return "";
  return url->name;
}

url0.h defines url->name as a char *.

But in _url_path_init we do:
  const char *name = url_to_string (url);

  mu_scheme_autodetect_p (name, &path);
  name = strdup (path);
  free (url->name);
  url->name = name;

I suspect those last three lines are best wrapped in a function that's
supposed to see url internals without the constness, but I want to make
sure before I touch the code.

Tks,
Jeff Bailey