Move / rename glob6.[1c] to glob.[1c]
Replace glob6 with glob
Add LIBEXECDIR, and change glob6 to glob
Update NOTES for sh6 and external utility changes
Update tests for sh6 and external utility changes
Change how sh6 finds its external utilities

And how its external utilities that invoke external utilities (glob,
fd2, if) find glob, fd2, goto, and if. NOTE: goto is a special case
since it invokes nothing. In short, PATH is no longer searched.
LIBEXECDIR is used instead. See NOTES and defs.h for further info.