include cleanups: remove unused headers and add feature test macros

clean up stdio_impl.h

this header evolved to facilitate the extremely lazy practice of
omitting explicit includes of the necessary headers in individual
stdio source files; not only was this sloppy, but it also increased
build time.

now, stdio_impl.h is only including the headers it needs for its own
use; any further headers needed by source files are included directly
where needed.

add bsd fgetln function

optimized to avoid allocation and return lines directly out of the
stream buffer whenever possible.