btrfs: build tags

Default to the same build behavior, but allow a go build tag to disable
building of the btrfs graphdriver

	go build -tags no_btrfs' ...
	$ go build
	$ objdump -S docker | grep btrfs | wc -l
	194
	$ go build -tags no_btrfs
	$ objdump -S docker | grep btrfs | wc -l
	1
	# that is a comment ;-)

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
btrfs: build tags

correct filename and make the tag more readable

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
packagers: btrfs build tag docs

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
typo

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)