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.h

@@ -97,6 +97,12 @@ gboolean ostree_sysroot_lock_finish (OstreeSysroot         *self,
 _OSTREE_PUBLIC
 void ostree_sysroot_unlock (OstreeSysroot  *self);
 
+_OSTREE_PUBLIC
+gboolean ostree_sysroot_init_osname (OstreeSysroot       *self,
+                                     const char          *osname,
+                                     GCancellable        *cancellable,
+                                     GError             **error);
+
 _OSTREE_PUBLIC
 gboolean ostree_sysroot_cleanup (OstreeSysroot       *self,
                                  GCancellable        *cancellable,