hush: reinstate cmd handling for NOMMU (with fat big warning).

hush: fix a case where none of pipe members could be started
  because of fork failure
hush: rename functions: xxx_real -> xxx
hush: try to add a bit more of vfork-friendliness
hush: add rudimentary design docs
hush: add TODO (newly discovered bug with globbing)

shell/hush_test/run-all

@@ -1,6 +1,9 @@
 #!/bin/sh
 
-test -x hush || { echo "No ./hush?!"; exit; }
+test -x hush || {
+    echo "No ./hush?! Perhaps you want to run 'ln -s ../../busybox hush'"
+    exit
+}
 
 PATH="$PWD:$PATH" # for hush and recho/zecho/printenv
 export PATH