ping: fix unaligned access to ancillary data

Use CMSG_FIRSTHDR rather than assume that an array is correctly aligned.
Fixes warnings on sparc64 and powerpcspe.

Submitted by: Ján Sučan <sucanjan@gmail.com>
MFH: 2 weeks
Sponsored by: Google LLC (Google Summer of Code 2019)
Differential Revision: https://reviews.freebsd.org/D21406