This patch removes the buffered I/O stream usage in process_handle_t and
its related utility functions. This simplifies the code and avoids racy
code where we used buffered I/O on non-blocking file descriptors.
See: https://bugs.torproject.org/21654
Background processes spawned by Tor now will have a valid stdin.
Pluggable transports can detect this behavior with the aformentioned
enviornment variable, and exit if stdin ever gets closed.
