gio-unix: Use EPOLL_CLOEXEC by default
First, there's no reason not to use the new epoll_create1 system call,
which quickly obsoleted epoll_create which has an obsolete and
unused size argument.
But more specifically, it offers EPOLL_CLOEXEC which we want
to use for general hygeine - there's no reason to potentially
leak this file descriptor to forked processes.
(GLib itself carefully closes file descriptors when forking child
processes, but it may be linked with other software that doesn't;
notably in my case for example the Rust standard library does not
do this and hence relies more on the application code using
O_CLOEXEC and variants)
This is just a drive-by fix; I saw the system call when I was using
strace to debug something else in rpm-ostree.