Add string valued expressions to ap_expr, do some API cleanup

- add possibility to have expressions that evaluate to a string and not to
  a boolean value
- modify ap_expr_parse_cmd() interface to support this and make it more
  convenient to use in general
- rename AP_EXPR_FLAGS_* to AP_EXPR_FLAG_* for consistency


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1142164 13f79535-47bb-0310-9956-ffa450edef68