traceroute6: fix capabilities for the rcv socket

On the receive socket, recvmsg() and poll()/select() is called.
Therefore, CAP_EVENT is needed in addition to CAP_RECV..

While there, check the socket for readbility before calling recvmsg().

Reviewed by: markj@
MFC after: 3 days
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D36878