dissect: split read-only flag into two

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.