setup with experimental libsolv-based dependency solving

[Jon Turney]

This has a lot of internal changes, so could use some wider testing. 
Please test.

   https://cygwin.com/setup/setup-2.882-41-g4cbfa1.x86.exe
   https://cygwin.com/setup/setup-2.882-41-g4cbfa1.x86_64.exe

Changes compared to 2.882:

User visible changes:

- 'Current' is replaced by 'Best' (which is slightly different in ways 
it's difficult to summarize briefly) and 'Sync' (which exposes the 
--force-current option through the UI).  These are modified by a 'Test' 
checkbox, which allows test packages to be used.
- For the handful of packages where the curr: version has a lower 
version number than some prev: (e.g. cscope), the higher version number 
is now preferred.

Internal changes:
- Uses the libsolv dependency solver, rather than a home-made one.
- Add support for 'depends: package (relation version) [...]', in a 
version section in setup.ini
- Add support for 'obsoletes:' in setup.ini, likewise

A big 'thank you' to Ken Brown for helping get this to the point where 
it's somewhat usable.