truss: Add support for utrace(2).

This uses the kdump(1) utrace support code directly until a common library
is created.

This allows malloc(3) tracing with MALLOC_CONF=utrace:true and rtld tracing
with LD_UTRACE=1. Unknown utrace(2) data is just printed as hex.

PR: 43819 [inspired by]
Reviewed by: jhb
MFC after: 2 weeks
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D3819