eal/linux: add function to allow interruptible epoll

eal/linux: add function to allow interruptible epoll

The existing definition of rte_epoll_wait retries if interrupted
by a signal. This behavior makes it hard to use rte_epoll_wait
for applications that want to use signals do do things like
exit polling loop and shutdown.

Since changing existing semantic might break applications, add
a new rte_epoll_wait_interruptible() function that does the
same thing as rte_epoll_wait but will return -1 and errno of EINTR
if it receives a signal.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Reviewed-by: Harman Kalra <hkalra@marvell.com>