hush: fix environment and memory leaks, add tests for them

function                                             old     new   delta
add_malloced_string_to_strings                         -     110    +110
run_list                                            1999    2086     +87
free_strings_and_unsetenv                              -      87     +87
hush_version_str                                       -      18     +18
pseudo_exec_argv                                     139     146      +7
static.version_str                                    17       -     -17
free_pipe                                            237     210     -27
done_word                                            790     642    -148
------------------------------------------------------------------------------
(add/remove: 3/1 grow/shrink: 2/2 up/down: 309/-192)          Total: 117 bytes

shell/hush_test/hush-vars/var_leaks.right

@@ -0,0 +1 @@
+OK

shell/hush_test/hush-vars/var_leaks.tests

@@ -0,0 +1,14 @@
+# external program
+a=b /bin/true
+env | grep ^a=
+
+# builtin
+a=b true
+env | grep ^a=
+
+# exec with redirection only
+# in bash, this leaks!
+a=b exec 1>&1
+env | grep ^a=
+
+echo OK