Build with -mno-red-zone on x86_64

Otherwise, an interrupt may overwrite part of the stack if we're in
checksum_stubs.c at the time. Should help with #80.

Also added -fno-stack-protector in case that's on by default somewhere.
Improve formatting of DHCP console logging, including printing out options.
Merge branch 'red-zone' of git://github.com/talex5/mirage-tcpip into 2.0.2-release
Sync changes for 2.0.2 release
Merge pull request #84 from avsm/2.0.2-release

2.0.2 release