mtcp: Make IPOutputStandalone() more generic

The IPOutputStandalone() should be capable of preparing packet
for any IPv4 protocol and not just TCP packet.

Signed-off-by: Anup Patel <anup@brainfault.org>
io_engine: netmap: Minor fixes to have netmap PS library working

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