Explicitly unlink the @link destination first.

Exposed by building multiarch + sngl, as sngl includes compatability
symlinks which are designed to be overwritten by packages.
Call /bin/bash rather than /bin/sh, helps sngl.
Add support for passing full PKGPATH's.

This primarily allows USER_ADDITIONAL_PKGS, and possibly SPECIFIC_PKGS,
to start working as expected.
Merge branch 'joyent/feature/miscfix/2012Q4' into joyent/release/2012Q4_multiarch
Merge branch 'joyent/feature/multiarch/2012Q4' into joyent/release/2012Q4_multiarch