[Toybox] [PATCH] Fix dmesg -c and -C.

[enh]

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...)
---
 lib/portability.h |  4 ++++
 toys/lsb/dmesg.c  | 44 ++++++++++++++++++++++++--------------------
 2 files changed, 28 insertions(+), 20 deletions(-)
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