traceroute6: Fix most warnings at the default WARNS level.

Fix some style issues as well. Leave -Wno-cast-aligned set for now, as
most of the warnings come casts of CMSG_DATA(), which does provide
sufficient alignment in practice.

Submitted by: Shubh Gupta <shubh@freebsd.org>
Sponsored by: Google (GSOC 2020)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D25603