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