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

apidoc/ostree-sections.txt

@@ -499,6 +499,7 @@ ostree_sepolicy_get_type
 OstreeSysroot
 ostree_sysroot_new
 ostree_sysroot_new_default
+ostree_sysroot_initialize
 ostree_sysroot_get_path
 ostree_sysroot_load
 ostree_sysroot_load_if_changed
@@ -508,6 +509,8 @@ ostree_sysroot_lock_async
 ostree_sysroot_lock_finish
 ostree_sysroot_unlock
 ostree_sysroot_unload
+ostree_sysroot_set_mount_namespace_in_use
+ostree_sysroot_is_booted
 ostree_sysroot_get_fd
 ostree_sysroot_ensure_initialized
 ostree_sysroot_get_bootversion