Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

dwatch(1): Allow `-E code' to override profile EVENT_DETAILS

This allows quick changes to the formatted output of a profile.

Sponsored by: Smule, Inc.

Fix display of wrong pid from dtrace_sched(4)

Fix a comment while here.

Sponsored by: Smule, Inc.

Introduce dwatch(1) as a tool for making DTrace more useful

Reviewed by: markj, gnn, bdrewery (earlier version)
Relnotes: yes
Sponsored by: Smule, Inc.
Differential Revision: https://reviews.freebsd.org/D10006