bug 157205: select() doesn't notify incoming stderr data, because stderr's
pipe isn't hooked up to the fileno() BufferedPipe. to fix, i added an "or"
pipe-event that can be triggered by either stdout or stderr, and hooked
them both up to fileno(). added a unit test for the bug and one for the
"or" pipe.
nail down select() on EOF: occasionally a channel would be closed remotely but select() wouldn't trigger. when a channel gets EOF or is closed, set the pipe FOREVER.
the previous windows pipe fix still didn't work. replace it with a new pipe.py abstraction of pipes (one for posix, one for windows) which appears to finally work on windows. for real this time. also add some more documentation to Channel to explain that after exec_command, invoke_shell, or invoke_subsystem, a Channel can't be reused.