avoid executing unsafe functions if we get a terminating signal while handling SIGCHLD; reject attempts to modify some attributes for readonly variables; more changes for unlocked stdio functions; quoting fixes for uncommon cases where IFS = ^A and the word is not being split

tests/dollar.right

@@ -742,3 +742,29 @@ argv[2] = <2>
 var=1 2
 argv[1] = <1 2>
 argv[1] = <1 2>
+argv[1] = <^Aaa^Abb^Acc^A--^Add^A>
+argv[1] = <^A--^A>
+ok 1
+ok 2
+ok 3
+ok 4
+ok 5
+ok 6
+ok 7
+ok 8
+ok 9
+ok 10
+ok 11
+ok 12
+ok 13
+ok 14
+ok 15
+ok 16
+ok 17
+ok 18
+ok 19
+ok 20
+ok 21
+ok 22
+ok 23
+ok 24