Bug#372712: Max-Pdiffs option

[Matthijs Kooijman <m.kooijman@student.utwente.nl>]

Hey,

I would propose a simple solution: Have a Acquire::Max-Pdiffs option. This is
the number of pdiffs that will be downloaded, if more are required, a full
download is done instead. The exact value would be dependent on the actual
connection, but it should be possible to specify a decent default. People with
really fast internet connections can tweak this option themselves (if it gets
documented, see #376029).

Also, this option might be used to replace Acquire::Pdiffs, for Max-Pdiffs = 0
would be always download full files, a special value "all" could be used to
always do Pdiffs. Later, a special value "auto" might be added when there is
some autodetecting mechanism that takes into account connection speed from
previous updates.

Gr.

Matthijs