merge traceroute6 into traceroute

[llfw]

this PR depends on #1099 (libipsec const-correctness) and #1100 (move traceroute into base). depending on what exactly we decide to commit in #1100, it may end up needing a rebase.

it is not ready for merging (a significant number of features don't work yet), but i wanted to submit it as a draft PR to validate the basic approach, which is explained in the commit message "traceroute: support IPv6".

a browsable version of the branch is available here: https://github.com/llfw/freebsd-src/tree/traceroute-merge/usr.sbin/traceroute

(if you'd prefer i not do this, do let me know.)