Support mounting /sysroot (and /boot) read-only

We want to support extending the read-only state to cover `/sysroot`
and `/boot`, since conceptually all of the data there should only
be written via libostree.  Or at least for `/boot` should *mostly*
just be written by ostree.

This change needs to be opt-in though to avoid breaking anyone.

Add a `sysroot/readonly` key to the repository config which instructs
`ostree-remount.service` to ensure `/sysroot` is read-only.  This
requires a bit of a dance because `/sysroot` is actually the same
filesystem as `/`; so we make `/etc` a writable bind mount in this case.

We also need to handle `/var` in the "OSTree default" case of a bind
mount; the systemd generator now looks at the writability state of
`/sysroot` and uses that to determine whether it should have the
`var.mount` unit happen before or after `ostree-remount.service.`

Also add an API to instruct the libostree shared library
that the caller has created a new mount namespace.  This way
we can freely remount read-write.

This approach extends upon in a much better way previous work
we did to support remounting `/boot` read-write.

Closes: https://github.com/ostreedev/ostree/issues/1265

src/boot/ostree-remount.service

@@ -22,12 +22,12 @@ DefaultDependencies=no
 ConditionKernelCommandLine=ostree
 OnFailure=emergency.target
 Conflicts=umount.target
-After=-.mount
+# Run after core mounts
+After=-.mount var.mount
 After=systemd-remount-fs.service
+# But we run *before* most other core bootup services that need write access to /etc and /var
 Before=local-fs.target umount.target
-# Other early boot units that need to write to /var
 Before=systemd-random-seed.service plymouth-read-write.service systemd-journal-flush.service
-# tmpfiles.d usually needs write access to a few places
 Before=systemd-tmpfiles-setup.service
 
 [Service]