convert xpg4/sh and ilk to use ksh93
xpg4/sh is apparently legally questionable, and certainly closed so we need to remove it and replace it with a link to ksh93. Plus, the installed code will be smaller that way.
Additionally, all the aliases (which are just shell scripts that execute the builtin via xpg4 sh), can now become links to /usr/bin/alias.
This reduces the total number of closed binaries we rely upon significantly.