capabilities: Add support for Linux capabilities(7)

This adds capabilities for start-stop-daemon by adding --capabilities
option. As a result, the user can specify the inheritable, ambient and
bounding set by define capabilities in the service script.

This fixes #314.

sh/start-stop-daemon.sh

@@ -53,6 +53,7 @@ ssd_start()
 		${error_log+--stderr} $error_log \
 		${output_logger_arg} \
 		${error_logger_arg} \
+		${capabilities+--capabilities} "$capabilities" \
 		${procname:+--name} $procname \
 		${pidfile:+--pidfile} $pidfile \
 		${command_user+--user} $command_user \

sh/supervise-daemon.sh

@@ -36,6 +36,7 @@ supervise_start()
 		${respawn_period:+--respawn-period} $respawn_period \
 		${healthcheck_delay:+--healthcheck-delay} $healthcheck_delay \
 		${healthcheck_timer:+--healthcheck-timer} $healthcheck_timer \
+		${capabilities+--capabilities} "$capabilities" \
 		${command_user+--user} $command_user \
 		${umask+--umask} $umask \
 		${supervise_daemon_args:-${start_stop_daemon_args}} \