eal/linux: add epoll wrappers

eal/linux: add epoll wrappers

The patch adds 'rte_epoll_wait' and 'rte_epoll_ctl' for async event wakeup.
It defines 'struct rte_epoll_event' as the event param.
When the event fds add to a specified epoll instance, 'eptrs' will hold
the rte_epoll_event object pointer.
The 'op' uses the same enum as epoll_wait/ctl does.
The epoll event support to carry a raw user data and to register a callback
which is executed during wakeup.

Signed-off-by: Cunming Liang <cunming.liang@intel.com>