inet.4 icmp.4 udp.4: Add missing sysctls, other fixes

Add missing sysctls to inet.4 and icmp.4, using references to ip.4
for variables and groups documented there. Add sysctl.8 to "see also"
and intro to variable section. Rename "MIB Variables" section to
"MIB (sysctl) Variables", as most people will associate with sysctl.
Revise history: the ICMP implementation was in 4.2BSD.

Reviewed by: manpages(pauamma)
Differential Revision: https://reviews.freebsd.org/D36003