extract method DRbObject.prepare_backtrace. add DRb.regist_server,
remove_server, fetch_server. change server in thread variable if
in-proc sesrver. [druby-ja:113]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_1_8@8206 b2dd03c8-39d4-4d8f-98ff-823fe69b080e