mod_proxy: Ensure we don't attempt to amend a table we are iterating
through, ensuring that all headers listed by Connection are removed.


mod_proxy, mod_proxy_http: Connection headers must be stripped on the way
in and out, support an optional function to handle this.


hunk 1: C89 please;
hunk 2: optional functions are usually declared static.


mod_proxy: Make sure we skip empty tokens when parsing the Connection
header.

Submitted by: minfrin, fuankg, minfrin
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1502201 13f79535-47bb-0310-9956-ffa450edef68