lib: Add ostree_sysroot_init_osname() API, bump mtime

And change the command line to use it.  rpm-ostree had a copy
of this code, and thus there's a clear reason to have an API.

While we're moving this into API, ensure the mtime on deploy is bumped
after an osname is created, so that daemons like rpm-ostree can notice
changes.  (In reality, creating the directory should do this, but
let's be double sure)

src/libostree/ostree-sysroot-deploy.c

@@ -1855,12 +1855,8 @@ ostree_sysroot_write_deployments (OstreeSysroot     *self,
                                 requires_new_bootversion ? "yes" : "no",
                                 new_deployments->len - self->deployments->len);
 
-  /* Allow other systems to monitor for changes */
-  if (utimensat (self->sysroot_fd, "ostree/deploy", NULL, 0) < 0)
-    {
-      glnx_set_prefix_error_from_errno (error, "%s", "futimens");
-      goto out;
-    }
+  if (!_ostree_sysroot_bump_mtime (self, error))
+    goto out;
 
   /* Now reload from disk */
   if (!ostree_sysroot_load (self, cancellable, error))