Swift caputures both sys.stdout and sys.stderr messages and
transfers them to syslog. However, whole of the messages
uses the "STDOUT:" prefix, currently.
It will be quite confusable for debugging purpose because we
cannot make sure whether "Swift didn't caputure stderr" or
"Swift captured stderr but didn't show it" when we want to log
some stderr messages.
This patch enables to allow a new prefix "STDERR:" and use it
for sys.stderr.
Change-Id: Idf4649e9860b77c3600a5cd0a3e0bd674b31fb4f
99a3d1a1c5