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/zbad2

@@ -0,0 +1,19 @@
+## TODO: fix and add to testsuite
+
+## # bash zbad2
+## ZVAR=z.map
+## *.map
+## # hush zbad2
+## ZVAR=z.map
+## z.map  <====== !!!
+
+## hush does globbing for "VAR=val" too!
+## it should do it only for non-assignments.
+## even if word looks like assignment, it can be non-assignemnt:
+## ZVAR=*.map /bin/echo ZVAR=*.map
+## ^dont_glob           ^glob
+
+>ZVAR=z.map
+ZVAR=*.map /bin/echo ZVAR=*.map
+ZVAR=*.map
+echo "$ZVAR"