Debian on raspberrypi: failed to configure wlan0

[Franco Martelli <martellif67@gmail.com>]

Hi everybody,

Following the instructions reported on the Debian unofficial port
home-site [1] I successful installed Debian on raspberrypi 3B 2016 all
works fine for my needs but configuring the built-in wi-fi interface
apparently it works but reporting errors:

~# ip addr show dev wlan0
3: wlan0: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN group
default qlen 1000
    link/ether b8:27:eb:8b:10:67 brd ff:ff:ff:ff:ff:ff

~# ifup wlan0
RTNETLINK answers: File exists
ifup: failed to bring up wlan0

the ifup command reports that it fails to bring up but the interface is
configured:

~# ip addr show dev wlan0
3: wlan0: <NO-CARRIER,BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc
pfifo_fast state DORMANT group default qlen 1000
    link/ether b8:27:eb:8b:10:67 brd ff:ff:ff:ff:ff:ff
    inet 192.168.0.9/24 brd 192.168.0.255 scope global wlan0
       valid_lft forever preferred_lft forever

if I try to de-configure the interface it fails:

~# ifdown wlan0
ifdown: interface wlan0 not configured

How can I de-configure wlan0 and why do I get errors when I bring up
with ifup command?
Some useful information about my configuration:

~# cat /etc/network/interfaces.d/wlan0
# allow-hotplug wlan0
iface wlan0 inet static
    address 192.168.0.9
    netmask 255.255.255.0
    network 192.168.0.0
    broadcast 192.168.0.255
    gateway 192.168.0.1
    wpa-conf /etc/wpa_supplicant/wpa_supplicant.conf

~# cat /etc/wpa_supplicant/wpa_supplicant.conf
ctrl_interface=/var/run/wpa_supplicant
update_config=1
network={
    ssid="myessid"
    psk=dfd452fedacd69d6d54582770dc93acebfb6f2ec2aac7d2e3f24e6ecacafc487
}

~# systemctl is-enabled wpa_supplicant
disabled

Thanks for any answer, best regards.

[1] https://salsa.debian.org/raspi-team/image-specs
-- 
Franco Martelli