Ruby

thread.c: hoist out timeval arithmetic functions

timeval arithmetic may be reused in other places and this
makes sleep_timeval easier-to-read.

• thread.c (timeval_add): hoist out of sleep_timeval
  (timeval_update_expire): ditto
  (sleep_timeval): use new functions

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@61301 b2dd03c8-39d4-4d8f-98ff-823fe69b080e