posix: implement readpipe using non-blocking I/O (issue4336)
On Linux, fstat().st_size of a pipe always returns 0, even if the
pipe has data available for reading. This meant that reading from
and subsequently printing the stderr pipe content after wireproto
commands over SSH meant that available data wasn't being printed.
We now implement pipe reading on POSIX by doing a non-blocking
read for all available data.
Greg Ward, author of the original bfiles extension
Na'Tosha Bard of Unity Technologies
Fog Creek Software
Special thanks to the University of Toronto and the UCOSP program