zshrc: correctly handle FreeBSD/OSX scenarios
zshrc: correctly handle FreeBSD/OSX scenarios

In the case of FreeBSD, no provision was made for the coreutils version
of ls(1) being present on the system, which should be handled in the
exact same way as on OSX.

For both operating systems, default to gls(1) if the GNU coreutils are
installed, instead of relying on PATH.

aelleth/dotfiles