* Allow override for the shared gem installation directory for
    rubygems packagers.

  * Lock gem cache files for read and write to improve thread safety.

  * Use io/console when available.

  * Minor cleanup.

* test/rubygems:  ditto.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44157 b2dd03c8-39d4-4d8f-98ff-823fe69b080e