truss: Make control message header parsing more robust

print_cmsg() was assuming that the control message chain is well-formed,
but that isn't necessarily the case for sendmsg(2). In particular, if
cmsg_len is zero, print_cmsg() will loop forever. Check for truncated
headers and try to recover if possible.

Reviewed by: tuexen
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35476