sys_cmd error handling improved.

(1) If the subprocess exits nonzero then the exit status
    is unpicked and logged.
(2) If the exec in the child fails, the command and
    errno string are written to stderr (which should
    end up in secnet's usual log output).
(3) _exit() is used instead of exit(), to avoid
    any possibility of craziness with stdio/atexit/etc.

Signed-off-by: Richard Kettlewell <rjk@terraraq.org.uk>