hush: finally make `cmd` safe on NOMMU

function                                             old     new   delta
generate_stream_from_string                            -     157    +157
expand_variables                                    2050    2003     -47
generate_stream_from_list                            139       -    -139
------------------------------------------------------------------------------
(add/remove: 1/1 grow/shrink: 0/1 up/down: 157/-186)          Total: -29 bytes

diff --git a/shell/hush_test/hush-misc/syntax_err.right b/shell/hush_test/hush-misc/syntax_err.right
index 08a270c..680e796 100644
--- a/shell/hush_test/hush-misc/syntax_err.right
+++ b/shell/hush_test/hush-misc/syntax_err.right
@@ -1,2 +1,4 @@
 shown
 hush: syntax error: unterminated '
+test
+not shown