Implement "dmesg -w".

Implement "dmesg -w".

Annoyingly, this means writing a new dmesg implementation that uses /dev/kmsg
rather than the klogctl system call. Worse, pre-3.5 kernels don't support
that, so we need to keep the old implementation around as long as we still
care about those kernels.

Since I'm here, add the fancy colors from modern dmesg and the -C flag.

Tested on Nexus 9's 3.10 kernel, an Ubuntu 3.13 kernel, and the
Nexus One's 2.6.35 kernel.