tcp.4: Sort sysctl variables

In preparation for updates including missing variables, sort the
sysctl variables in the MIB variables section alphabetically.
Add a new "hostcache" entry for the hostcache node, containing the
intro text that was previously in hostcache.enable. Also cleanups
per review comments.

Reviewed by: transport(tuexen), manpages(bcr)
Differential Revision: https://reviews.freebsd.org/D35844
MFC after: 1 week

(cherry picked from commit 5cf709ce72c0b6eb4b4d57db015a65f8a84166d5)