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.

platform: implement readpipe() Reading all available data from a pipe has a platform-dependent implementation. This patch establishes platform.readpipe() by copying the inline implementation in sshpeer.readerr(). The implementations for POSIX and Windows are currently identical. The POSIX implementation will be changed in a subsequent patch.

exchange: remove duplicated addition to pushop.stepdone Not sure how it got there but it is useless.