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-repo-private.h

@@ -93,6 +93,7 @@ struct OstreeRepo {
   int objects_dir_fd;
   int uncompressed_objects_dir_fd;
   GFile *sysroot_dir;
+  GWeakRef sysroot; /* Weak to avoid a circular ref; see also `is_system` */
   char *remotes_config_dir;
 
   GHashTable *txn_refs;