timeout.rb: watcher thread name

* lib/timeout.rb (Timeout#timeout): set watcher thread name to
  caller location for debugging.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@53195 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/timeout.rb

@@ -73,11 +73,13 @@ module Timeout
   def timeout(sec, klass = nil)   #:yield: +sec+
     return yield(sec) if sec == nil or sec.zero?
     message = "execution expired".freeze
+    from = "from #{caller_locations(1, 1)[0]}" if $DEBUG
     e = Error
     bl = proc do |exception|
       begin
         x = Thread.current
         y = Thread.start {
+          Thread.current.name = from
           begin
             sleep sec
           rescue => e