Introduce wp_allowed_protocols() for use in wp_kses() and esc_url(). See #18268.

This allows plugins to filter the list of protocols used for esc_url() too, and helps us keep the list of protocols in sync.


git-svn-id: https://core.svn.wordpress.org/trunk@18826 1a063a9b-81f0-0310-95a4-ce76da25c4cd