Created attachment 152425 [details]
rc.d/tcpmss script and documentation

This PR supplies new rcNG startup script to ease configuration and use of ng_tcpmss(4) netgraph node with ipfw(8) rules for custom TCP MSS adjustments.
It contains the script itself and patches for defaults/rc.conf and rc.conf.5 manual page. Description from the manual:

     tcpmss_enable
                 (bool) Set to ``NO'' by default.  Setting this to ``YES''
                 enables configuration of ng_tcpmss(4) netgraph nodes to per-
                 form custom TCP MSS adjustments using ipfw(8) rules. The
                 tcpmss_instances variable must also be set then. Kernel mod-
                 ules netgraph(4), ng_ipfw(4), and ng_tcpmss(4) will be loaded
                 if the kernel was not built with corresponding options
                 NETGRAPH, options NETGRAPH_IPFW, and options NETGRAPH_TCPMSS.

     tcpmss_instances
                 (str) Set to the list of ng_tcpmss(4) instances to configure
                 on this host. Values of the list are used to form instance
                 names and as netgraph cookies, so they should be numbers. A
                 tcpmss_<number> variable is assumed to exist for each value
                 of the list.  The value of this variable is used as maxMSS to
                 configure the ng_tcpmss(4) instance.

                 To adjust MSS to corresponding maxMSS value configured with
                 tcpmss_<number> variable one can use ipfw(8) rules like:

                 netgraph <number> tcp from any to any setup in

     tcpmss_verbose
                 (bool) Set to ``YES'' to enable verbose ng_tcpmss(4) configu-
                 ration messages.