packages: add package for NTP

Reviewed by: imp, manu
Pull Request: https://github.com/freebsd/freebsd-src/pull/1193

Remove $FreeBSD$: one-line sh pattern

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

Simplify/improve idioms in usr.sbin/ntp Makefiles

- Use SRCTOP-relative paths to other directories instead of
.CURDIR-relative ones. This simplifies pathing in make/displayed output.
- Also, use :H where possible/sensical to manipulate .CURDIR-relative
paths
- Remove superfluous bsd.own.mk .includes which are already handled via
src.opts.mk .includes

MFC after: 1 week
Sponsored by: Dell EMC Isilon