dtc: re-apply r353961, r354115

I missed in final review of r357923's diff that these ones hadn't yet been
sent upstream and inadvertently reverted them. =-( Re-apply now.

usr.bin/dtc/dtb.cc

@@ -36,6 +36,7 @@
 #include <sys/types.h>
 #include <inttypes.h>
 #include <stdio.h>
+#include <stdlib.h>
 #include <unistd.h>
 #include <errno.h>
 

usr.bin/dtc/fdt.cc

@@ -1563,11 +1563,11 @@ device_tree::parse_file(text_input_buffer &input,
 {
 	input.next_token();
 	// Read the header
-	if (input.consume("/dts-v1/;"))
+	while (input.consume("/dts-v1/;"))
 	{
 		read_header = true;
+		input.next_token();
 	}
-	input.next_token();
 	if (input.consume("/plugin/;"))
 	{
 		is_plugin = true;