update, and make better use of, login and logout url functions

[wnorris]

I've attached two diffs to this ticket. The first updates the wp_login_url and wp_logout_url functions in two ways. It makes use of add_query_arg() to ensure query parameters are escaped, and it adds a new filter at the end of each method so that plugins can replace the default URL location. You can already do this with site_url, but it requires a lot more work to skip over all the calls to site_url that you don't care about. This allow plugins to hook right in where they need to.

The second diff updates a number of other core files to actually make use of the wp_login_url and wp_logout_url methods. This includes the two included themes, comment templates, new user emails, and a few other places.

All changes should be fully backwards compatible.

[ryan]

(In [10931]) Updates to wp_login_url() and wp_logout_url(). Use them in more places. Props wnorris. fixes #9536