io_engine: netmap: Use shared memory for control data

There are lot of network applications that use multi-processing
instead of multi-threadind.

To support multi-processing applications, we pslib should be
initialized only once in parent process and all processes
including parent process should do mtcp_init() separtely.

This patch moves all control data to shared memory so that
it can be shared between multiple processes.

Signed-off-by: Anup Patel <anup@brainfault.org>
io_engine: netmap: Get poll msecs from environment variable

Signed-off-by: Anup Patel <anup@brainfault.org>