[patch] EDSP 0.5 w/ multi-arch support for external solvers

[Stefano Zacchiroli <zack@debian.org>]

Heya, I've prepared a set of patches that implement (and document)
version 0.5 of the EDSP protocol, which is used by APT to talk to
external solvers.  Most notably, EDSP 0.5 add support for multi-arch,
and will allow external solvers to propose proper solutions also for
users that use foreign architectures.

Each patch includes changes to the code and the corresponding
documentation snippet for doc/external-dependency-solver-protocol.txt

On the other hand, the patches do not touch debian/changelog (to
minimize the risk of conflicts), which is yet to be amended.

Can you please merge these patches and/or let me know which changes I
should apply to make them suitable for merge? I can also open a
wishlist bug report for this, if you want me to.

Here is a comprehensive diffstat:

$ git diff debian/sid --stat 
 apt-pkg/edsp.cc                             | 32 +++++++++++++++++++++++++++++---
 doc/external-dependency-solver-protocol.txt | 33 ++++++++++++++++++++++++++-------
 2 files changed, 55 insertions(+), 10 deletions(-)

Many thanks in advance,
Cheers.

PS I'd appreciate explicit Cc: on replies, as I'm not subscribed to the
   deity@ list