Facility: prepend each line with a facility id.
NoFacility: for one-module scripts.
StderrRedirector: capture stderr output to log as facility 'stderr'.
redirect_stderr: install it.
set_logger: new fun.
log: main interface.

This is an updated patch from pull request #16 which will throw an
exception if coro is called without starting the event_loop(). The
main update is to bring it up to date with recent changes and make
the linux poller throw the correct exception in this case.

flag readv() as not implemented.