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-private.h

@@ -98,6 +98,9 @@ gboolean _ostree_sysroot_query_bootloader (OstreeSysroot     *sysroot,
                                            GCancellable      *cancellable,
                                            GError           **error);
 
+gboolean _ostree_sysroot_bump_mtime (OstreeSysroot *sysroot,
+                                     GError       **error);
+
 typedef enum {
   OSTREE_SYSROOT_CLEANUP_BOOTVERSIONS = 1 << 0,
   OSTREE_SYSROOT_CLEANUP_DEPLOYMENTS  = 1 << 1,