hush: support "! cmd | cmd" negation

function                                             old     new   delta
done_word                                            749     791     +42
run_list                                            1821    1859     +38
checkjobs                                            334     351     +17
done_pipe                                             61      74     +13
static.reserved_list                                 132     144     +12
initialize_context                                    53      45      -8
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 5/1 up/down: 122/-8)            Total: 114 bytes

shell/hush_test/hush-parsing/negate.right

@@ -0,0 +1,35 @@
+! printing !
+0
+1
+1
+0
+0
+0
+!
+a
+b
+c
+! 1
+a 1
+b 1
+c 1
+! 1
+a 1
+b 1
+c 1
+0
+0
+0
+0
+1
+1
+1
+1
+0
+0
+0
+0
+1
+1
+1
+1

shell/hush_test/hush-parsing/negate.tests

@@ -0,0 +1,16 @@
+echo ! printing !
+! false
+echo $?
+! true
+echo $?
+if ! false; then false; echo $?; fi
+echo $?
+if ! false; then ! false; echo $?; fi
+echo $?
+for a in ! a b c; do echo $a; done
+for a in ! a b c; do ! echo -n "$a "; echo $?; done
+for a in ! a b c; do ! /bin/echo -n "$a "; echo $?; done
+for a in ! a b c; do ! echo -n "$a " | false; echo $?; done
+for a in ! a b c; do ! echo -n "$a " | true; echo $?; done
+for a in ! a b c; do ! { echo -n "$a " | false; }; echo $?; done
+for a in ! a b c; do ! { echo -n "$a " | true; }; echo $?; done