Use proper convention for relative path linking

The change made in e835ee6 did not
follow the convention for relative path symlinks. Per the convention,
paths be prefixed with `${BINDIR}`, not `/bin/`. `${BINDIR}` can be
modified from the Makefile, on the command line, or in the
environment. This convention is the canonically correct way to do
things.

This follows the convention used in `bin/pkill/Makefile`,
`bin/timeout/Makefile`, etc.

MFC after:	1 week
MFC with:	f05948d
MFC with:	e835ee6
Requested by:	jrtc27, kevans

bin/cpuset/Makefile

@@ -3,6 +3,6 @@ PROG=   cpuset
 
 LIBADD=	jail
 
-SYMLINKS+=	../../bin/cpuset	/usr/bin/cpuset
+SYMLINKS+=	../..${BINDIR}/cpuset	/usr/bin/cpuset
 
 .include <bsd.prog.mk>