parse.y: Traverse AST post parsing to detect break/next usage issues

Seemed easiest to do it via the traversal infrastructure as we do not
otherwise track enough context/scope when instantiating break or next
statements.

Might be worth moving this out of src/parse.y, but didn't exactly know
where. Or maybe we wait until there's more such trivial validations
popping up

Fixes #2440

testing/btest/language/next-break-context-errors.zeek

@@ -0,0 +1,85 @@
+# @TEST-DOC: Check break and next usage within for, while, switch and hooks.
+
+# @TEST-EXEC-FAIL: zeek -b %INPUT
+# @TEST-EXEC: TEST_DIFF_CANONIFIER=$SCRIPTS/diff-remove-abspath btest-diff .stderr
+function f()
+	{
+	next;
+	}
+
+event zeek_init() { f(); };
+
+@TEST-START-NEXT
+function f()
+	{
+	break;
+	}
+
+event zeek_init() { f(); };
+
+@TEST-START-NEXT
+event zeek_init()
+	{
+	next;
+	}
+
+@TEST-START-NEXT
+event zeek_init()
+	{
+	break;
+	}
+
+@TEST-START-NEXT
+event zeek_init()
+	{
+	if ( T )
+		break;
+	}
+
+@TEST-START-NEXT
+event zeek_init()
+	{
+	local history = "Sr";
+	switch history {
+		case "S":
+			print history;
+			next;
+			break;
+	}
+	}
+
+@TEST-START-NEXT
+global the_hook: hook(c: count);
+
+hook the_hook(c: count)
+	{
+	next;
+	}
+
+@TEST-START-NEXT
+global the_hook: hook(c: count);
+
+hook the_hook(c: count)
+	{
+	if ( T )
+		next;
+	}
+
+@TEST-START-NEXT
+# Should report 3 errors.
+global the_hook: hook(c: count);
+
+hook the_hook(c: count)
+	{
+	next;
+	}
+
+event zeek_init()
+	{
+	break;
+	}
+
+event zeek_init()
+	{
+	next;
+	}