Unicode uses file names with explicit versions for beta publication.
This commit introduces a variable on the makefile level to distinguish
between beta and regular versions of file names.
 
common.mk: Define new variable UNICODE_BETA, usually set to NO,
but would be YES during tests with beta data files. Pass the value
of this variable to tool/downloader.rb with option --unicode-beta.

tool/downloader.rb: Receive and store value of --unicode-beta.
Raise an exception if value is YES, because we don't yet actually
deal with this case. Continue as usual if value is not YES.

This completes the changes needed in the makefile. Upcomming changes
will only affect tool/downloader.rb.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66448 b2dd03c8-39d4-4d8f-98ff-823fe69b080e