setup with experimental libsolv-based dependency solving

[Jon Turney]

On 30/10/2017 15:52, Jon Turney wrote:
> On 25/10/2017 20:18, Jon Turney wrote:
>>
>> This has a lot of internal changes, so could use some wider testing. 
>> Please test.
[...]
> I've replaced these with:
[...]

I've replaced these with:

      https://cygwin.com/setup/setup-2.882-62-g75a2e0.x86.exe
      https://cygwin.com/setup/setup-2.882-62-g75a2e0.x86_64.exe

Changes compared to setup-2.882-49-gcc8b05:

libsolv related:
- Fix crash after incomplete download
- Handle going backwards to mirror selection correctly

other:
- Improve behaviour after download error
- Don't fatal() on unexpected early window messages
- Make 'System Proxy Settings' the default, rather than 'Direct'
- Remove support from installing from a local directory which doesn't 
contains a setup.ini file

I'm kind of tempted to tag this, stick a fork in it, and call it done, 
but I don't think we are quite there yet...