deploy: Allow extensions to write to Morph log file

This is achieved by passing it the write end of a pipe, so that the
extension has somewhere to write debug messages without clobbering either
stdout or stderr.

Previously deployment extensions could only display status messages on
stdout, which is good for telling the user what is happening but is not
useful when trying to do post-mortem debugging, when more information is
usually required.

This uses the asyncore asynchronous event framework, which is rather
specific to sockets, but can be made to work with file descriptors, and
has the advantage that it's part of the python standard library.

It relies on polling file descriptors, but there's a trick with pipes to
use them as a notification that a process has exited:

1.  Ensure the write-end of the pipe is only open in the process you
    want to know when it exits
2.  Make sure the pipe's FDs are set in non-blocking read/write mode
3.  Call select or poll on the read-end of the file descriptor
4.  If select/poll says you can read from that FD, but you get an EOF,
    then the process has closed it, and if you know it doesn't do that
    in normal operation, then the process has terminated.

It took a few weird hacks to get the async_chat module to unregister
its event handlers on EOF, but the result is an event loop that is
asleep until the kernel tells it that it has to do something.

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