DHCP: Support MTU option

This patch introduces support for MTU option as described in
RFC2132, chapter 5.1. Interface MTU Option

Amazon EC2 networking uses this option in some cases and it gives
throughput improvement of about 250% on big instances with 10G networking.

Netperf results for hi1.4xlarge instances, TCP_MAERTS test, OSv runs netserver:

Send buffer size     Throughput w/ patch (Mbps)     Throughput w/o patch (Mbps)     Improvement (%)

32                   4912.29                        1386.28                         254
64                   4832.01                        1385.99                         249
128                  4835.09                        1401.46                         245
256                  4746.41                        1382.28                         243
512                  4849.04                        1375.23                         253
1024                 4631.8                         1356.69                         241
2048                 4859.59                        1371.92                         254
4096                 4864.99                        1383.67                         252
8192                 4627.07                        1364.05                         239
16384                4868.73                        1366.48                         256
32768                4822.69                        1366.63                         253
65536                4837.67                        1353.87                         257

Signed-off-by: Dmitry Fleytman <dmitry@daynix.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>