Optimized @{*-core/*-test/*-os}.

For efficiency, such parcel now reduces all OS testers (e.g., :is_os_linux()) to
unconditionally report either success or failure as appropriate. Added
:define_function_if() to @{*-core/*-define/function/test}, a generalization of
:define_function_if_pathable() defining a function in terms of the first of an
arbitrary set of conditional tests to succeed. As a rousing show of force,
:is_display_server() is now defined via :define_function_if().