Add nginx detection to the Permalink Settings screen.

Introduces got_url_rewrite() and a corresponding filter, which should now be used in lieu of the got_rewrite filter in got_mod_rewrite().

This does not write or even suggest nginx configuration; rather, it prevents nginx from being considered as either Apache or as an unrecognized server.

props johnbillion.
fixes #25098.


git-svn-id: https://develop.svn.wordpress.org/trunk@25456 602fd350-edb4-49c9-b593-d223f7449a82