icmp.4 inet.4 udp.4: sort sysctl variables

Sort the sysctl(3)/sysctl(8) variables in the MIB Variables section
alphabetically. This is in preparation for adding missing variables
(at least in inet.4 and icmp.4). A few other touchups suggested in
review.

Reviewed by: bcr
Differential Revision: https://reviews.freebsd.org/D35843
MFC after: 1 week

(cherry picked from commit 3b656d465127de066511b6ffd02fb9fef85c7a53)