remote: use strvecs to store remote url/pushurl

remote: use strvecs to store remote url/pushurl

Now that the url/pushurl fields of "struct remote" own their strings, we
can switch from bare arrays to strvecs. This has a few advantages:

  - push/clear are now one-liners

  - likewise the free+assigns in alias_all_urls() can use
    strvec_replace()

  - we now use size_t for storage, avoiding possible overflow

  - this will enable some further cleanups in future patches

There's quite a bit of fallout in the code that reads these fields, as
it tends to access these arrays directly. But it's mostly a mechanical
replacement of "url_nr" with "url.nr", and "url[i]" with "url.v[i]",
with a few variations (e.g. "*url" could become "*url.v", but I used
"url.v[0]" for consistency).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>