ofpbuf: Enable ofpbuf_push() to expand headroom.

ofpbuf: Enable ofpbuf_push() to expand headroom.

Until now, ofpbuf_put() has been able to reallocate an ofpbuf to
expand the tailroom, but ofpbuf_push() has not been able to expand
the headroom in an analogous way.  This omission is simply because
it has never been useful to do this.  However, an upcoming change
will make the amount of headroom in an ofpbuf passed up from a
dpif to ofproto harder to predict.  It seems that we might as well
simply implement expanding headroom, so this commit does that.

Acked-by: Jesse Gross <jesse@nicira.com>