Define a cursor defface; minor face optimizations.

Define a cursor defface; minor face optimizations.

* faces.el (face-spec-reset-face): Reset all attributes in one
single call to set-face-attribute.
(face-spec-match-p): Make it a defsubst.
(frame-set-background-mode): New arg KEEP-FACE-SPECS.
(x-create-frame-with-faces, tty-create-frame-with-faces)
(tty-set-up-initial-frame-faces): Don't recompute face specs in
frame-set-background-mode, since they are recomputed immediately
afterwards in face-set-after-frame-default.
(face-set-after-frame-default): Minor optimization.
(cursor): Provide non-trivial defface spec.

* custom.el (custom-theme-recalc-face): Simplify.