Remove $FreeBSD$: one-line sh pattern

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

WARNS is now default to 6.

Import stdbuf(1) and the shared library it relies on.
This tool changes the default buffering behaviour of standard
stdio streams.

It only works on dynamic binaries. To make it work for static
ones it would require cluttering stdio because there no single
entry point.

PR: 166660
Reviewed by: current@, jhb
Approved by: kib (mentor)
MFC after: 1 week