traceroute6: Fix Capsicum rights for rcvsock

• Always use distinct sockets for send and recv
• Limit rights on the recv socket

For ICMP6 we were using the same socket for both send and receive, and
we limited rights on the socket such that it's impossible to receive
anything.

PR: 254623
Diagnosed by: Zhenlei Huang <zlei.huang@gmail.com>
Reviewed by: oshogbo
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D29523