As ksh93 is the real ksh devleoped by AT&T, git the user the option
of installing ksh as ${LOCALBASE}/bin/ksh instead of as
${LOCALBASE}/bin/ksh93. If the KSH option is chosen, this port will
conflict with shells/pdksh. By default the binary that is installed is
ksh93 (conflicting with shells/ksh).

In the future, we might want to add a little ports infrastructure
allowing ports that need a ksh to depend on one of the various ksh or
ksh clones in ports.