Add minibuffer input support to commands that set the frame size

Add minibuffer input support to commands that set the frame size

* lisp/frame.el (set-frame-property--interactive): Internal function to
produce the interactive form of `set-frame-width' and
`set-frame-height'.  Offer the current size as default.  (Bug#9970)
* src/frame.c (Fset_frame_height): Modify to call
`set-frame-property--interactive'.
(Fset_frame_width): Modify to call `set-frame-property--interactive'.
* doc/lispref/frames.texi (Frame Size): Update the manuals.
* etc/NEWS: Advertise the change (bug#9970).