Constify pointers in ap_expr lookup tables, as well as the already

Constify pointers in ap_expr lookup tables, as well as the already
const pointed-to strings.  ~1/2Kb moves to r/o text section, size(1)
diff:

    text           data     bss     dec     hex filename
- 667519          18384   13952  699855   aadcf httpd
+ 668015          17864   13952  699831   aadb7 httpd

* server/util_expr_eval.c
  [*_var_names, expr_provider_multi]: Mark pointers in name
  lists as const.
  (core_expr_lookup): Adjust accordingly.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1877350 13f79535-47bb-0310-9956-ffa450edef68