max_start_pad: calculate globally, not via client graph

max_start_pad: calculate globally, not via client graph

There is quite a lot of plumbing of max_start_pad values from one
place to another.  Sadly this plumbing is not complete, which can lead
to crashes as the start padding is exhausted.  And completing it is a
lot of work which would be difficult to get correct, even if that's
possible in principle.

Instead, we take a different approach.  We calculate a single global
max_pad_start value that can be used everywhere.  It is the sum of:

 * Headers that "site" instances might need add to the start of
   packets (source and destination site indices, message type both
   inside and outside the encryption;

 * Anything that "transform" instances might need to add to the start
   of packets.  This depends on the transforms, but since it isn't a
   priori predictable which path any particular incoming packet might
   take, we have to take the worst case.

   These transform instances are applied only by "site" and each
   packet goes through at most on "forward" transform instance.

 * Anything that "comm" instances might need to add.  This is
   currently only needed for the proxy feature.

This is based on the assumption that a packet may follow one of these
paths:

    comm -->- site_incoming --.         ,-- site_outgoing -->- comm
                               \       /
                                netlink
                               /       \
    tun/slip ----->-----------'         `------>---------- tun/slip

On the inbound side, tun and slip have to set up the buffer with the
necessary start padding.

site_incoming only removes stuff from the beginning (since it uses
transform->reverse).  netlink doesn't add or remove anything.

It is site_outgoing and comm which may need to prepend things.  site
additionally calls transform->forwards.

If in the future a module appears which can take packets out of the
RHS of this diagram and feed them back into the left, it may have to
do something about the buffer.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>