collector: freopen /proc/kmsg in write mode

freopen("/proc/kmsg", "a", stderr) => -EBADF
...but...
freopen("/proc/kmsg", "w", stderr) => a valid FILE *

I'm guessing that this fails in the append case because the
underlying lseek call fails because /proc/kmsg is of length zero,
or otherwise because it is not an on-disk file.