Ruby

• thread.c: add Thread#backtrace_locations' method. This method is similart to caller_locations' method for
  specific method.
  And fix to accept level' and n' parameters for Thread#backtrace' and Thread#backtrace_locations'.
  caller' (and caller_locations') do not return caller' method frame. However, Thread#backtrace' (and Thread#backtrace_locations') return Thread#backtrace' method frame itself
  if `Thread.current.backtrace' was called.
• vm_backtrace.c: ditto.
• internal.h: ditto.
• test/ruby/test_backtrace.rb: add tests.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@37716 b2dd03c8-39d4-4d8f-98ff-823fe69b080e