Remove $FreeBSD$: one-line sh pattern

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

cp: Adjust the sparse file tests.

* The sparsity check was ineffective: it compared the apparent size in bytes to the actual size in blocks. Instead, write a tool that reliably detects sparseness.
* Some of the seq commands were missing an argument.
* Based on empirical evidence, 1 MB holes are not necessarily large enough to be preserved by the underlying filesystem. Increase the hole size to 16 MB.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: cracauer
Differential Revision: https://reviews.freebsd.org/D38414

cp: add some basic tests

There are some tests available in the NetBSD test suite, but we don't
currently pass all of those; further investigation will go into that. For
now, just add a basic test as well as a test that copies from /dev/null to a
file.

The /dev/null test confirms that the file gets created if it's empty, then
that it truncates the file if it's non-empty. This matches some usage that
was previously employed in the build and was replaced in r366042 by a
simpler shell construct.

I will also plan on coming back to expand these in due time.

MFC after: 1 week