Completed canonical file descriptor refactoring.

is_stdin_readable_sans_blocking() now behaves substantially as expected
under real-world operating conditions (e.g., with slight, imperceptible
latency to account for heavy load). In return, a new cadre of
set_*_to_stdin_pipe() functions have been split from their nascent
set_*_to_stdin() brethren *OMITTING* such dubious functionality.