[Email-SIG] policy patch review

[R. David Murray]

OK, so I've started designing/coding the more interesting stuff (the new
header class), and I find I need a new policy control.  So what I'd like
to do is commit the policy stuff I've done to this point so I can use that
as a base for further development.  So I've removed the parameters that
I'm not ready to implement yet and have posted a version of the patch
for final review on the tracker (http://bugs.python.org/issue11731).
This version of the patch pretty much just adds the hooks for maxlinelen,
compared to the previous version.

The proposed policy patch shows all the essentials of the API/framework,
with several "hooked up" examples.  It does not include any
existing-parameter deprecations, I'll leave that for much later.

Please let me know what you think.  Absent negative feedback I'll probably
commit this (and the folding algorithm rewrite) early next week.

--David