Add filter (with cancel ability) to wp_redirect() function

[markjaquith]

Patch adds a filter to the $location in wp_redirect. If $location is false after the filter, the function aborts, which means that a filter would be able to cancel a given redirect.

While writing a plugin today, I happened upon a circumstance where this would have been very useful.

[markjaquith]

Patch for trunk

[ryan]

Fine by me.

[markjaquith]

(In [4407]) Add wp_redirect filter, with cancel ability. fixes #3250