Add a notion of "physical" sysroot, use for remote writing

Using `${sysroot}` to mean the physical storage root: We don't want to write to
`${sysroot}/etc/ostree/remotes.d`, since nothing will read it, and really
`${sysroot}` should just have `/ostree` (ideally). Today the Anaconda rpmostree
code ends up writing there. Fix this by adding a notion of "physical" sysroot.
We determine whether the path is physical by checking for `/sysroot`, which
exists in deployment roots (and there shouldn't be a `${sysroot}/sysroot`).

In order to unit test this, I added a `--sysroot` argument to `remote add`.
However, doing this better would require reworking the command line parsing for
the `remote` argument to support specifying `--repo` or `--sysroot`, and I
didn't quite want to do that yet in this patch.

Closes: https://github.com/ostreedev/ostree/issues/892

Closes: #896
Approved by: jlebon

src/libostree/ostree-sysroot.c

@@ -135,6 +135,8 @@ ostree_sysroot_constructed (GObject *object)
   repo_path = g_file_resolve_relative_path (self->path, "ostree/repo");
   self->repo = ostree_repo_new_for_sysroot_path (repo_path, self->path);
   self->repo->is_system = TRUE;
+  /* Hold a weak ref for the remote-add handling */
+  g_weak_ref_init (&self->repo->sysroot, object);
 
   G_OBJECT_CLASS (ostree_sysroot_parent_class)->constructed (object);
 }
@@ -813,6 +815,26 @@ ostree_sysroot_load_if_changed (OstreeSysroot  *self,
                                cancellable, error))
     return FALSE;
 
+  /* Determine whether we're "physical" or not, the first time we initialize */
+  if (!self->loaded)
+    {
+      /* If we have a booted deployment, the sysroot is / and we're definitely
+       * not physical.
+       */
+      if (self->booted_deployment)
+        self->is_physical = FALSE;  /* (the default, but explicit for clarity) */
+      /* Otherwise - check for /sysroot which should only exist in a deployment,
+       * not in ${sysroot} (a metavariable for the real physical root).
+       */
+      else if (fstatat (self->sysroot_fd, "sysroot", &stbuf, 0) < 0)
+        {
+          if (errno != ENOENT)
+            return glnx_throw_errno_prefix (error, "fstatat");
+          self->is_physical = TRUE;
+        }
+      /* Otherwise, the default is FALSE */
+    }
+
   self->bootversion = bootversion;
   self->subbootversion = subbootversion;
   self->deployments = deployments;