* process.c (proc_detach): new method Proc#detach(pid) which
  create background watcher thread to issue waitpid. [new]

* process.c (rb_detach_process): utility function to detach
  process from C code.

* ext/pty/pty.c (pty_finalize_syswait): terminate watcher thread,
  and detach child process (by creating new idle waitpid watcher
  thread).

* ext/pty/pty.c (pty_syswait): may lost signal stopped child.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@3561 b2dd03c8-39d4-4d8f-98ff-823fe69b080e