Cope with glibc too old to have epoll_create1().

Cope with glibc too old to have epoll_create1().

Commit fa31b6f4e supposed that we didn't have to worry about that
anymore, but it seems that RHEL5 is like that, and that's still
a supported platform.  Put back the prior coding under an #ifdef,
adding an explicit fcntl() to retain the desired CLOEXEC property.

Discussion: https://postgr.es/m/12307.1493325329@sss.pgh.pa.us