Restore full 'bin/package test' functionality

package now uses mamake to run all available regression tests
(currently iffe, mamake and ksh93) instead of just the ksh tests.

However, as a consequence, passing options or other arguments to
the shtests script via bin/package is no longer possible -- run
bin/shtests directly for that.

src/Mamfile, src/*/Mamfile:
- Make the 'test' virtual target execute subdirectories by
  including the 'install' and 'all' virtual targets within it.

src/cmd/ksh93/Mamfile:
- Simplify and update the script in the 'test' virtual target.

src/cmd/builtin/Mamfile:
- Add dummy 'test' target to avoid an error.

bin/package, src/cmd/INIT/package.sh:
- Run 'mamake test' from the arch/*/src directory. This is the one
  that must be the initial working directory for mamake, though the
  Mamfiles aren't here; mamake finds them via the VPATH variable.
- Update self-doc.

src/cmd/builtin/Mamfile

@@ -67,3 +67,5 @@ make install
 		exec - fi
 	done ${INSTALLROOT}/bin virtual
 done install virtual
+make test
+done test dontcare virtual