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.

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git-svn-id: http://core.svn.wordpress.org/trunk@25377 1a063a9b-81f0-0310-95a4-ce76da25c4cd