Implement pseudo-value 'reset' of face attrributes

Implement pseudo-value 'reset' of face attrributes

* doc/lispref/display.texi (Face Attributes):
* etc/NEWS: Document the new pseudo-value 'reset'.

* src/xfaces.c (realize_named_face, lookup_derived_face)
(gui_supports_face_attributes_p, lookup_named_face)
(Finternal_merge_in_global_face, merge_named_face, merge_faces):
Handle the 'reset' pseudo-value of a face's attribute.
(syms_of_xfaces): New symbol 'reset'.
(RESET_P): New macro.
(check_lface_attrs, Finternal_set_lisp_face_attribute): Allow
'reset' as a value of any attribute except ':inherit'.
(Bug#38771)