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/lib/Mamfile

@@ -9,8 +9,10 @@ note * documented in Glenn Fowler's paper "A Make Abstract Machine":
 note * http://web.archive.org/web/20041227143022/http://www2.research.att.com/~gsf/mam/mam.html
 note *
 note component level :MAKE: equivalent
-make install
-	make all
-		exec - ${MAMAKE} -r '*' ${MAMAKEARGS}
-	done all virtual
-done install virtual
+make test
+	make install
+		make all
+			exec - ${MAMAKE} -r '*' ${MAMAKEARGS}
+		done all virtual
+	done install virtual
+done test virtual