Handle /dev/stdout and /dev/stderr as streams.

There is a legit use-case for logging to stdout/stderr: If supervisor is
run on a platform like Heroku that only supports capturing stdout/stderr
(but no logfiles or syslog), applications started by it should at least
have a chance at logging something if supervisord is run as a foreground
process.

This could be achieved by passing /dev/stdout and /dev/stderr as the
respective logfiles, but these may not be opened or rotated. This simple
patch detects if one of these files has been specified and chooses a
StreamHandler instead of a FileHandler to service them.