sysroot: Add an API to lock

If a system administrator happens to type `ostree admin upgrade`
multiple times, currently that will lead to a potentially corrupted
system.

I originally attempted to do locking *internally* in `libostree`, but
that didn't work out because currently a number of the commands
perform multi-step operations that all need to be serialized.  All of
the current code in `ostree admin deploy` is an example.

Therefore, allow callers to perform locking, as most of the higher
level logic is presently implemented there.

At some point, we can revisit having internal locking, but it will be
difficult.  A more likely approach would be similar to Java's approach
with concurrency on iterators - a "fail fast" method.

src/libostree/ostree-sysroot.h

@@ -62,6 +62,9 @@ char *ostree_sysroot_get_deployment_dirpath (OstreeSysroot    *self,
 
 GFile * ostree_sysroot_get_deployment_origin_path (GFile   *deployment_path);
 
+gboolean ostree_sysroot_lock (OstreeSysroot  *self, GError **error);
+void ostree_sysroot_unlock (OstreeSysroot  *self);
+
 gboolean ostree_sysroot_cleanup (OstreeSysroot       *self,
                                  GCancellable        *cancellable,
                                  GError             **error);