IPv4: experimental changes to allow net 0/8, 240/4, part of 127/8

Combined changes to allow experimentation with net 0/8 (network 0),
240/4 (Experimental/"Class E"), and part of the loopback net 127/8
(all but 127.0/16). All changes are disabled by default, and can be
enabled by the following sysctls:

net.inet.ip.allow_net0=1
net.inet.ip.allow_net240=1
net.inet.ip.loopback_prefixlen=16

When enabled, the corresponding addresses can be used as normal
unicast IP addresses, both as endpoints and when forwarding.

Add descriptions of the new sysctls to inet.4.

Add <machine/param.h> to vnet.h, as CACHE_LINE_SIZE is undefined in
various C files when in.h includes vnet.h.

The proposals motivating this experimentation can be found in

https://datatracker.ietf.org/doc/draft-schoen-intarea-unicast-0
https://datatracker.ietf.org/doc/draft-schoen-intarea-unicast-240
https://datatracker.ietf.org/doc/draft-schoen-intarea-unicast-127

Reviewed by: rgrimes, pauamma_gundo.com; previous versions melifaro, glebius
Differential Revision: https://reviews.freebsd.org/D35741