Ruby

thread.c: reset waitq of keeping mutexes in child

We must not maintain references to threads in the parent process
in any mutexes held by the child process.

• thread_sync.c (rb_mutex_cleanup_keeping_mutexes): new function
• thread.c (rb_thread_atfork): cleanup keeping mutexes
  [ruby-core:85940] [Bug #14578]

Fixes: r58604 (commit 3586c9e0876e784767a1c1adba9ebc2499fa0ec2)
("reduce rb_mutex_t size from 160 to 80 bytes on 64-bit")

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62668 b2dd03c8-39d4-4d8f-98ff-823fe69b080e