Use err instead of errno in wavy_listen
Check EWOULDBLOCK on result of ::accept

c.f.) http://linuxjm.sourceforge.jp/html/LDP_man-pages/man2/accept.2.html
Do not raise error when accept is failed
Sleep a while when failed to accept
Merge pull request #17 from gintenlabo/fix-jubatus-issue-717

Fix behavior when ::accept is failed (Fixes jubatus issue 717)