stack: Fix sed syntax for freebsd.

	Freebsd version of sed requires an empty argument for
	-i <suffix>.
pkg: Add missing libffi dependency.
create-stack-user: Add CPATH to sudo.

	CPATH is an additional include path for clang.
	By default, FreeBSD10's clang has only /usr/include.
	It needs to be extended to /usr/local/include, too.
	CPATH needs to be propagated to sudo commands for
	installing python packages.
stack: Add CPATH global variable for freebsd.

	FreeBSD requires CPATH=/usr/local/include .
	Otherwise clang does not include headers from there.
stack: Fix missing fi keyword.