Use proper convention for relative path linking

The change made in e835ee68e13361b841c983fa4a49dd6c19dcdec4 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: f05948d4e98d3abd0965a2994e9e42add6908ff3
MFC with: e835ee68e13361b841c983fa4a49dd6c19dcdec4
Requested by: jrtc27, kevans