hush: fix mishandling of a'b'c=fff as assignments. They are not.

function                                             old     new   delta
parse_stream                                        1920    2004     +84
done_word                                            715     752     +37
parse_and_run_stream                                 328     333      +5
builtin_exec                                          25      29      +4
pseudo_exec_argv                                     138     139      +1
run_list                                            2006    1999      -7
is_assignment                                        215     134     -81
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 5/2 up/down: 131/-88)            Total: 43 bytes

shell/hush_test/hush-misc/assignment1.right

@@ -0,0 +1,9 @@
+if1:0
+while1:0
+until1:0
+if2:0
+while2:0
+until2:0
+if3:0
+while3:0
+until3:0

shell/hush_test/hush-misc/assignment1.tests

@@ -0,0 +1,42 @@
+# Assignments after some keywords should still work
+
+if a=1 true; then a=1 true; elif a=1 true; then a=1 true; else a=1 true; fi
+echo if1:$?
+while a=1 true; do a=1 true; break; done
+echo while1:$?
+until a=1 false; do a=1 true; break; done
+echo until1:$?
+
+if a=1 true
+ then a=1 true
+ elif a=1 true
+ then a=1 true
+ else a=1 true
+ fi
+echo if2:$?
+while a=1 true
+ do a=1 true
+ break
+ done
+echo while2:$?
+until a=1 false
+ do a=1 true
+ break
+ done
+echo until2:$?
+
+if
+ a=1 true; then
+ a=1 true; elif
+ a=1 true; then
+ a=1 true; else
+ a=1 true; fi
+echo if3:$?
+while
+ a=1 true; do
+ a=1 true; break; done
+echo while3:$?
+until
+ a=1 false; do
+ a=1 true; break; done
+echo until3:$?

shell/hush_test/hush-misc/assignment2.rigth

@@ -0,0 +1,2 @@
+hush: can't exec 'a=b': No such file or directory
+1

shell/hush_test/hush-misc/assignment2.tests

@@ -0,0 +1,4 @@
+# This must not be interpreted as an assignment
+a''=b true
+echo $?
+# (buglet: $? should be 127. it is currently 1)