Fix dmesg -c and -C.

Fix dmesg -c and -C.

I never use these, so I didn't notice I'd broken them until someone who
does bringup complained.

The "one weird trick" with SEEK_DATA is documented at the URL we already
point to. SEEK_DATA was added in Linux 3.1 (2011) and isn't available in
glibc 2.19 (2014), so I've added that to "portability.h" for the benefit
of Ubuntu 14.04.

Also make -c and -C mutually exclusive.

Also fix some of the formatting I introduced earlier. (A clang-format file
would help prevent these mistakes...)