Fix minor fallout from sysctl strings being nulterminated now. The dmesg
code can read the buffer via sysctl or from a core file. In the core file
case there will be no nulterm, and the code copes with that, but now in the
sysctl case there is a nulterm (there didn't used to be). The least
disruptive way to restore the old behavior (and eliminate a spurious '\000'
at the end of the output) is to remove the nulterm (by decrementing the
buffer length) in the sysctl case.