mtcp: Depend on environment variable instead for configuration

This patch makes mTCP depend upon multiline environment variables for
configuration, which are:
MTCP_GENERAL_CONFIG - General Configuration
MTCP_ROUTE_CONFIG - Route Table Configuration
MTCP_ARP_CONFIG - ARP Table Configuration

This is first attempt to reduce dependency of mTCP on config files.

Enventually, we will also remove MTCP_ROUTE_CONFIG and MTCP_ARP_CONFIG
so that we get initial ARP table and Route table via syscalls.

Signed-off-by: Anup Patel <anup@brainfault.org>
mtcp: Forward more packets host stack using ps_slowpath_packet()

The mTCP only processes TCP packets and ARP packets. We also
need to keep host ARP table in sync with mTCP ARP table.

This patch ensures that we process ARP packet in both mTCP and
host stack. Further, we also ensure that all UDP packets are
forwarded to host stack for ensuring proper functioning of
DHCP client.

Signed-off-by: Anup Patel <anup@brainfault.org>
io_engine: netmap: Poll both NIC ring and host ring in ps_recv_chunk()

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