Fix epoll_ctl erroring out

Fix epoll_ctl erroring out

Do not call epoll_ctl on FDs representing actual files,
which causes the call to error out with EPERM.

This use of this mechanism was twofold; first, pipes
need to actually be added to epoll (to see if they're
full). The second use was to discover whether a descriptor
represents a file. This information is known a priori,
which removes the need to make calls doomed to fail.

Change-Id: Ic11c211ee88e70899998e794847b79afc0a90d9a
Signed-off-by: Michal Bloch <m.bloch@samsung.com>