mod_proxy: Don't limit the size of the connectable Unix Domain Socket paths.

Since connect() to UDS path is used at several places, introduce
ap_proxy_connect_uds() in proxy_util.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1602989 13f79535-47bb-0310-9956-ffa450edef68