More ap_expr updates:

- minor bump for ap_expr_exec_re() introduced in r1037504
- actually commit the changes to allow using backreferences in SetEnvIfExpr
- automatically add the correct entries to the Vary-header if the result of
  an expression evaluation depends on a request header
  (can be turned off by setting the AP_EXPR_FLAGS_DONT_VARY flag)
- set AP_EXPR_FLAGS_DONT_VARY in mod_log_config's conditional logging
- fix various off-by-one errors in req_table_func


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1037540 13f79535-47bb-0310-9956-ffa450edef68