s6-svscanboot: launch s6-svscan with notify=fd:4

Now that the bug that made it fail is fixed, use the
start-stop-daemon readiness notification mechanism when
launching s6-svscan.

(We do not run s6-svscan under supervise-daemon because that
would needlessly lengthen the supervision chain. Yo dawg,
we heard you like supervisors.)

Author: Laurent Bercot

init.d/s6-svscan.in

@@ -18,7 +18,7 @@ command_args="$RC_SVCDIR"
 command_background=yes
 pidfile=/var/run/s6-svscan.pid
 umask=022
-# notify=fd:4  # when notify=fd is fixed, uncomment here and add -d4 in svscanboot
+notify=fd:4
 
 depend() {
 	need localmount

sh/s6-svscanboot.sh.in

@@ -17,7 +17,6 @@ s6-log -bpd3 -- t s1048576 n10 "$logdir"
 EOF
 chmod 0755 "$scandir/s6-svscan-log/run"
 
-# when notify=fd is fixed, add -d4 after -X3
 exec redirfd -wnb 1 "$scandir/s6-svscan-log/fifo" \
 fdmove -c 2 1 \
-s6-svscan -X3 -- "$scandir" 0</dev/null 3>/dev/console
+s6-svscan -X3 -d4 -- "$scandir" 0</dev/null 3>/dev/console