Permit zero value for 'child-frame-border-width' parameter (Bug#46184)

Permit zero value for 'child-frame-border-width' parameter (Bug#46184)

* doc/lispref/frames.texi (Layout Parameters): Update entry on
'child-frame-border-width' parameter.
* src/frame.c (make_frame): Init child_frame_border_width to -1.
(Fframe_child_frame_border_width): Return internal border width if
child frame border width parameter is nil.
(gui_report_frame_params): Report nil as child frame border
width parameter if the frame value is negative.
* src/frame.h (FRAME_INTERNAL_BORDER_WIDTH): Return value of
child frame border width only if it is not negative.
* src/xfns.c (Fx_create_frame): Default child frame border to -1
when recording it in its frame slot via gui_default_parameter.
* src/nsfns.m (ns_set_child_frame_border_width): Handle nil ARG.
(Fx_create_frame): Default child frame border width parameter to
nil.
* src/w32fns.c (w32_set_child_frame_border_width): Handle nil ARG.
(Fx_create_frame): Default child frame border width parameter to
nil.
* src/xfns.c (x_set_child_frame_border_width): Handle nil ARG.
(Fx_create_frame): Default child frame border width parameter to
nil.