mksquashfs: add -action_file as synonym for -af

Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>
diff --git a/squashfs-tools/mksquashfs.c b/squashfs-tools/mksquashfs.c
index 4ee45b1..7c4b6e3 100644
--- a/squashfs-tools/mksquashfs.c
+++ b/squashfs-tools/mksquashfs.c
@@ -5230,9 +5230,11 @@
 			if(res == 0)
 				exit(1);
 
-		} else if(strcmp(argv[i], "-af") == 0) {
+		} else if(strcmp(argv[i], "-action_file") == 0 ||
+				strcmp(argv[i], "-af") ==0) {
 			if(++i == argc) {
-				ERROR("%s: -af missing filename\n", argv[0]);
+				ERROR("%s: %s missing filename\n", argv[0],
+							argv[i - 1]);
 				exit(1);
 			}
 			if(read_action_file(argv[i], 0) == FALSE)