Revive MASTER_SITES.
Upgrade to version 2.0:
- Canonicalize x86_64 to amd64.
- Canonicalize i86pc to i386.
- Implement a -format option in favor of the plethora of element selectors.
- Add -group, -release and -relgroup.
- Use lsb_release(1) on systems with Linux kernels to determine operating
  system name (use the distribution ID).
- Handle Fedora, Mandriva, RedHat and SuSE.
- Handle Mac OS X.
- Handle Cygwin (at least on XP).

(kim)