initctl: add support for --json to list command

This allows for easy access to the *disabled* services:

    root@anarchy:~# initctl ls --json |jq '.available - .enabled'
    [
      "chronyd.conf",
      "dnsmasq.conf",
      "gdbserver.conf",
      "inadyn.conf",
      "inetd.conf",
      "isisd.conf",
      "lldpd.conf",
      "mstpd.conf",
      "ntpd.conf",
      "ospf6d.conf",
      "ospfd.conf",
      "querierd.conf",
      "ripd.conf",
      "ripng.conf",
      "sshd.conf",
      "syslogd.conf",
      "telnetd.conf",
      "uftpd.conf",
      "wpa_supplicant.conf",
      "zebra.conf"
    ]

As discussed in PR issue #335

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

src/initctl.h

@@ -36,6 +36,7 @@ extern int icreate;			/* initctl -c */
 extern int iforce;			/* initctl -f */
 extern int ionce;			/* initctl -1 */
 extern int heading;			/* initctl -t */
+extern int json;			/* initctl -j */
 extern int verbose;			/* initctl -v */
 extern int plain;			/* initctl -p */
 extern int quiet;			/* initctl -q */