ap_expr: follow up to r1810605.

The "split" and "join" operators are now a prefix, ala perl.
Add the "sub" operator for string substitutions, prefix still.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1811104 13f79535-47bb-0310-9956-ffa450edef68