RFC1078 tcpmux help

help is reserved keyword according to rfc1078 and must list tcpmux
services

This is an implementation of the help command for the tcpmux-server
builtin service included in xinetd.

It also introduces new flag TCPMUXHELP which is required for the tcpmux
service to be published via the help command.  TCPMUX and TCPMUXPLUS
services which do not have the TCPMUXHELP flag are not listed in order
to preserve the existing behavior and to support providing unpublished
tcpmux services.