hush: fix multimple dependent variable expansion cases

function                                             old     new   delta
get_local_var_value                                  100     171     +71
expand_assignments                                    46      76     +30
reset_traps_to_defaults                              229     238      +9
maybe_set_to_sigexit                                  47      50      +3
init_sigmasks                                        211     214      +3
builtin_trap                                         462     465      +3
expand_vars_to_list                                 2412    2408      -4
run_pipe                                            1568    1533     -35
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 6/2 up/down: 119/-39)            Total: 80 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

shell/hush_test/hush-vars/var_serial.right

@@ -0,0 +1,5 @@
+Assignments only: c=a
+Assignments and a command: c=a
+Assignments and a builtin: c=a
+Assignments and a function: c=a
+Done

shell/hush_test/hush-vars/var_serial.tests

@@ -0,0 +1,22 @@
+a=a
+
+b=b
+c=c
+# Second assignment depends on the first:
+b=$a c=$b
+echo Assignments only: c=$c
+
+b=b
+c=c
+b=$a c=$b "$THIS_SH" -c 'echo Assignments and a command: c=$c'
+
+b=b
+c=c
+b=$a c=$b eval 'echo Assignments and a builtin: c=$c'
+
+b=b
+c=c
+f() { echo Assignments and a function: c=$c; }
+b=$a c=$b f
+
+echo Done