rgw: move url escaping to a common place

rgw: move url escaping to a common place

This is useful outside of the s3 interface. Rename url_escape()
url_encode() for consistency with the exsting common url_decode()
function. This is in preparation for the next commit, which needs
to escape url-unsafe characters in another place.

Backport: dumpling
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>