Fix epoch-related panic in ipdivert, ensuring in_broadcast() is called
within epoch.

Simplify gigantic div_output() by splitting it into 3 functions,
handling preliminary setup, remote "ip[6]_output" case and
local "netisr" case. Leave original indenting in most parts to ease
diff comparison. Indentation will be fixed by a followup commit.

Reported by: Nick Hibma <nick at van-laarhoven.org>
Reviewed by: glebius
Differential Revision: https://reviews.freebsd.org/D23317