setup 2.904 release candidate - please test

[Jon Turney]

A new setup release candidate is available at:

   https://cygwin.com/setup/setup-2.904.x86_64.exe (64 bit version)
   https://cygwin.com/setup/setup-2.904.x86.exe    (32 bit version)

Please test, and report any problems here.

This is not the place for setup feature requests.

Changes compared to 2.903:

- The '--disable-old-keys' option is still accepted, but is now the default.

- Add an '--enable-old-keys' option, for if you really need to install 
from an old mirror snapshot, for some reason.

- The Start Menu folder used for 32-bit installs on WoW64 is now named 
'Cygwin (32-bit)', to avoid collision with the name used for 64-bit 
installs.

- Fix a memory allocation error which could (very rarely) cause crashes 
during package installation.