listen to multiple sockets; squash-merge PR #13 consisting of commits:

commit 532c403db4ee12e73d1c37c5b1adb8f2d2c6cd3d
Author: ttakezawa <ttakezawa@users.noreply.github.com>
Date:   Thu Apr 10 11:52:23 2014 +0900

    refactoring of accept_loop

commit 187b8006798ae6f47795b94a31b535ee3b1a11f9
Author: ttakezawa <ttakezawa@users.noreply.github.com>
Date:   Wed Apr 9 11:16:25 2014 +0900

    select is not used if only one socket is specified.

commit f4aaf9b993c95fb52dd6a65b5a818ee024d063ef
Author: ttakezawa <ttakezawa@users.noreply.github.com>
Date:   Mon Mar 24 16:32:53 2014 +0900

    Each worker process accepts the multiple sockets with select(2).