Add HARDWARE section of urnis(4) driver to try to gain more visibility.

MFC after:	3 days
Have dtrace(1) handle SIGPIPE by cleaning up and exiting. Additionally,
install signal handlers when running in list mode (-l), and acknowledge
interrupts by cleaning up and exiting. This ensures that a command like

$ dtrace -l -P 'pid$target' -p <target PID> | less

won't cause the ptrace(2)d target process to be killed if less(1) exits
before all dtrace output is consumed.

Reported by:		Anton Yuzhaninov <citrin+bsd@citrin.ru>
Differential Revision:	https://reviews.freebsd.org/D880
Reviewed by:		rpaulo
MFC after:		1 month
Sponsored by:		EMC / Isilon Storage Division