fs,epoll: Add ability to call kcmp to find target files

Hi! This is slightly updated series for epoll extension in procfs and kcmp syscall. The procfs patch and manpage is unchanged while kcmp syscall is slightly updated to drop hard to read if()'s and shrink memory pressue with kcmp_epoll_slot argument.

Please take a look, once time permit. Being playing with this patches via criu directly, works as expected.

https://lists.openvz.org/pipermail/criu/2017-February/036031.html