dissect: split read-only flag into two

Let's have one flag to request that when dissecting an image the
loopback device is made read-only, and another one to request that when
it is mounted to make it read-only. Previously both concepts were always
done read-only together.

(Of course, making the loopback device read-only but mounting it
read-write doesn't make too much sense, but the kernel should catch that
for us, no need to make restrictions from our side there)

Use-case for this: in systemd-repart we'd like to operate on images for
adding partitions. Thus we'd like to have the loopback device writable,
but if we read repart.d/ snippets from it, we want to do that read-only.

src/dissect/dissect.c

@@ -770,7 +770,7 @@ static int run(int argc, char *argv[]) {
 
         r = loop_device_make_by_path(
                         arg_image,
-                        FLAGS_SET(arg_flags, DISSECT_IMAGE_READ_ONLY) ? O_RDONLY : O_RDWR,
+                        FLAGS_SET(arg_flags, DISSECT_IMAGE_DEVICE_READ_ONLY) ? O_RDONLY : O_RDWR,
                         FLAGS_SET(arg_flags, DISSECT_IMAGE_NO_PARTITION_TABLE) ? 0 : LO_FLAGS_PARTSCAN,
                         &d);
         if (r < 0)