Pick up default selection color on OSX when user defaults are not set.

Pick up default selection color on OSX when user defaults are not set.

* lisp/faces.el (region): Change ns_selection_color to
ns_selection_fg_color, add ns_selection_bg_color.

* src/nsterm.h (NS_SELECTION_BG_COLOR_DEFAULT): Renamed from
NS_SELECTION_COLOR_DEFAULT.
(NS_SELECTION_FG_COLOR_DEFAULT): New.

* src/nsterm.m (ns_selection_color): Remove.
(ns_get_color): Check for ns_selection_(fg|bg)_color using
NSColor selectedText(Background)Color.  Only for COCOA.
(ns_term_init): Remove assignment of ns_selection_color, logic
moved to ns_get_color.