Cygwin: select: set pipe writable only if PIPE_BUF bytes left

Cygwin: select: set pipe writable only if PIPE_BUF bytes left

Linux select(2) returns the pipe as writable if at least one
free page (4K onl most systems) is left in a page-oriented buffer
handling. This is the same as PIPE_BUF.

Emulate this behaviour by only returning the pipe as writable
if at least 4K space is left in the buffer.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>