[ccan] RFC822 module (attempt 3)

[David Gibson]

Here's my third attempt at an rfc822 module for ccan.  In this one I
fixed the bug in rfc822_body() which was pointed out, I've updated to
a newer ccanlint and found a bunch of problems with that, and added
some extra tests to improve coverage.

The one remaining warning from ccanlint is a license incompatibility -
I've nominated LGPL 2.1+, but I depend on foreach which is LGPL3+.  I
only use foreach in the testcases, not the module code itself, so I
don't know if this is a showstopper or not.