inetd(8): Provide HTTP proxy example using netcat

[debdrup]

Summary

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.

Test Plan

Ran igor, only got some probably-erroneous feedback.

[debdrup]

Use -N to close nc again after the file has EOF'd.

This addresses feedback given by AllanJude on IRC.

[debdrup]

Add comment explaining what the example achieves, add nc(1) to SEE ALSO section

Partially based on further feedback from AllanJude on IRC.

[lwhsu]

Don't forget update .Dd :-)

[debdrup]

Fix .Dd nit and nc path nit.

Pointed out by lwshu and kevans respectively.