Coord: Use epoll in place of select.

This is needed to make coordinator scalable. Typically, the select
system call can handle upto 1024 file descriptors only while epoll
allows us to handle several thousand fds.

git-svn-id: svn://svn.code.sf.net/p/dmtcp/code/trunk@2306 ba6dc0ae-232f-0410-883f-9ebac64fc565