inetd(8): Provide HTTP proxy example using netcat

One of the fortunes that are included in freebsd-tips talks about how
the superserver can be used to proxy connections with netcat, but there are
no examples provided. This commit adds an example with comment explaining
what it does.

Submitted by: debdrup
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D24800