setup 2.885 release candidate - please test

[Jon Turney <jon dot turney at dronecode dot org dot uk>]

A new setup release candidate is available at:

  https://cygwin.com/setup/setup-2.885.x86.exe    (32 bit version)
  https://cygwin.com/setup/setup-2.885.x86_64.exe (64 bit version)

Since this contains many internal changes, I think this could use some 
wider testing before being deployed. Please test and report any problems 
here.

This is not the place for setup feature requests.

Changes compared to 2.884:

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.
- The "prereq" page showing dependencies which will be added is replaced 
by "problems" page showing problems found by the dependency solver, with 
default solutions.
- A "confirm" page is added showing all the changes which will be made.

Internal changes:
- Uses the libsolv dependency solver, rather than a home-made one.
- Add support for 'depends2: package (relation version) [...]', in a 
version section in setup.ini
- Add support for 'obsoletes:' in setup.ini, likewise
- Add support for 'replace-versions:' in a package section setup.ini, to 
indicate problematic versions.

Other:
- Query the user for action to take if a corrupt local file is found
- Validate package hash after download
- Any MessageBox shown during setup.ini parsing is now modal

A big 'thank you' to Ken Brown for all his work on this.