ip.7: Add "Special and reserved addresses" section

ip.7: Add "Special and reserved addresses" section

Break out the discussion of special and reserved IPv4 addresses into
a subsection, formatted as a pair of definition lists, and briefly
describing three cases in which Linux no longer treats addresses
specially, where other systems do or did.

Also add a specific example to the NOTES paragraph that discourages
the use of IP broadcasting, so people can more easily understand
what they are supposed to do instead.

Suggested-by: John Gilmore <gnu@toad.com>
Signed-off-by: Seth David Schoen <schoen@loyalty.org>
Signed-off-by: Alejandro Colomar <alx@kernel.org>