sysroot: Handle ro /boot but rw /sysroot

sysroot: Handle ro /boot but rw /sysroot

The recent change in https://github.com/coreos/fedora-coreos-config/pull/659
broke some of our tests that do `mount -o remount,rw /sysroot` but
leave `/boot` read-only.

We had code for having `/boot` read-only before `/sysroot` but
in practice we had a file descriptor for `/sysroot` that we opened
before the remount that would happen later on.

Clean things up here so that in the library, we also remount
`/boot` at the same time we remount `/sysroot` if either are readonly.

Delete the legacy code for remounting `/boot` rw if we're not in
a mount namespace.  I am fairly confident most users are either
using the `ostree` CLI, or they're using the mount namespace.