[New Port] sysutils/dwatch-pwsnoop

DTrace module for FreeBSD dwatch(1) to read password prompts by snooping on
read/write syscalls.

Reviewed by: mat (mentor; earlier version), imp (mentor), dbaio
Approved by: imp (mentor)
Sponsored by: Smule, Inc.
Differential Revision: https://reviews.freebsd.org/D15415