Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Teach kdump to decode linux syscalls names too.

Fix bug introduced in my previous commit: the kernel always dump native
signal numbers, so no need to check the ABI in ktrpsig().

Suggested by: jhb
MFC after: 1 Month.