xmlrpc: include UNIX mbox 'From ' header in patch_get_mbox()

xmlrpc: include UNIX mbox 'From ' header in patch_get_mbox()

This function is misleadingly named 'mbox'; the contents do not begin
with a proper 'From xxx <date>' separator line. (Notably, the mbox
format does not have an authoritative standard, but at least this basic
'separator' construct is noted in http://tools.ietf.org/html/rfc4155.)

The Message.as_string() function takes an optional [unixfrom] boolean
argument. Let's use it, like we do everywhere else (including in the web
interface 'mbox' link).

Among other things, this means that we can straightforwardly concatenate
the output of patch_get_mbox(), and more tools can use this output
as-is.

Example header:

>From patchwork Fri Nov 21 18:24:29 2014
...

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>