use rb_pid_t instead of pid_t.

* process.c (rb_execarg_addopt, run_exec_pgroup): use rb_pid_t
  instead of pid_t.

* ext/pty/pty.c (raise_from_check, pty_check): ditto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39868 b2dd03c8-39d4-4d8f-98ff-823fe69b080e