Add relative URIs containing a query in esc_url()

[hakre]

I'd like to enhance the esc_url function to accept an url starting with ? (query) to not require a protocol.

That's comparable with the # (fragment) case we already have in.

This is useful to create links to the current page that need a query only. Otherwise the php file must be added and for frontend pages it won't work with pretty permalinks.

First approach

Related: (In [11126]) Don't prepend http to fragments. Props scohoust. fixes #4051