Print error message.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Catch signal and remove socket in the file systems.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Daemonize.

PID file can be specified by -p option.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>