Remove duplicated code

We preivously had two copies of ::BytesAvailable with only trivial
differences between them, and fixes have been applied to only one of
them.

Instead of duplicating the whole function, hide the FD_SET differences
behind a macro.  This leaves only one small __APPLE__-specific #if
block, and fixes ^C on non-__APPLE__ platforms.

llvm-svn: 210592