setup release candidate

setup release candidate

[Jon Turney]

Since this changes the file download mechanism, I think this could use 
some wider testing before being deployed.  Please test.

   https://cygwin.com/setup/setup-2.878.x86.exe
   https://cygwin.com/setup/setup-2.878.x86_64.exe

Changes compared to 2.877:

- "Direct Connection" now uses the WinINet API to fetch URLs.

This enables HTTPS and FTPS protocol support, and caching of mirrors.lst 
and setup.ini.

The existing, hand-built URL fetching, which only supports HTTP and FTP, 
is still available by choosing "Direct (legacy)".

- Fixes to "Use Internet Explorer Proxy Settings" mode

Progress is now reported for downloads.

Unwanted caching in "Temporary Internet Files" of package archives has 
been fixed.
(Addresses: https://cygwin.com/ml/cygwin/2016-09/msg00403.html)

A bug causing a long delay before downloading setup.ini from package 
repositories without a compressed setup.ini has been fixed.

- Add --allow-unsupported-windows option

Don't check the windows version.

The Cygwin mirror list is not read, so a URL (e.g. a Cygwin time machine 
circa) should be given with --site or via the GUI.

- Remove support for some long-obsolete setup.ini syntax (omitted 
version:, omitted size and checksum, various undocumented tokens)

- Remove cruft