Trying to bring up any wireless interface on a network with WPA auth
fails in a very peculiar way: the interface gets correctly configured,
including WPA auth, DHCP manages to get an IP, and then an error about
failure to bring up wpa_supplicant causes the interface to be brought
down.

wpa_supplicant: /sbin/wpa_supplicant daemon failed to start
run-parts: /etc/network/if-pre-up.d/wpasupplicant exited with return code 1
ifup: failed to bring up ***

Debugging `ifup -v wlan=***` gives the following log:
--- /etc/network/interfaces.d/***:
iface *** inet dhcp
	test wireless essid ***
	dns-search ***
	dns-nameserver 192.168.0.2 192.168.0.1
	wpa-essid ***
	wpa-psk ***

(and several other similar ones)

--- /etc/network/interfaces.d/wlan:
mapping wlan0
	script /usr/sbin/guessnet-ifupdown
	map verbose: true
	map syslog: true
	map default: none
	map iwscan-tries: 5
	map *** *** *** *** ***

--- up and down scripts installed:
/etc/network/if-down.d:
total 12
-rwxr-xr-x 1 root root 1015 Apr 13  2015 avahi-autoipd
-rwxr-xr-x 1 root root  372 Jul  4  2016 openvpn
-rwxr-xr-x 1 root root  256 Jul 20  2013 resolvconf
lrwxrwxrwx 1 root root   32 Aug  8 15:59 wpasupplicant -> ../../wpa_supplicant/ifupdown.sh

/etc/network/if-post-down.d:
total 4
lrwxrwxrwx 1 root root   23 Oct 10  2018 avahi-daemon -> ../if-up.d/avahi-daemon
lrwxrwxrwx 1 root root   29 Jan 28  2019 bridge -> /lib/bridge-utils/ifupdown.sh
lrwxrwxrwx 1 root root   25 Aug  8 15:59 hostapd -> ../../hostapd/ifupdown.sh
-rwxr-xr-x 1 root root 1409 Mar 24  2016 wireless-tools
lrwxrwxrwx 1 root root   32 Aug  8 15:59 wpasupplicant -> ../../wpa_supplicant/ifupdown.sh

/etc/network/if-pre-up.d:
total 12
lrwxrwxrwx 1 root root   29 Jan 28  2019 bridge -> /lib/bridge-utils/ifupdown.sh
-rwxr-xr-x 1 root root  344 Jan 28  2014 ethtool
lrwxrwxrwx 1 root root   25 Aug  8 15:59 hostapd -> ../../hostapd/ifupdown.sh
-rwxr-xr-x 1 root root 4191 Sep 15  2018 wireless-tools
lrwxrwxrwx 1 root root   32 Aug  8 15:59 wpasupplicant -> ../../wpa_supplicant/ifupdown.sh

/etc/network/if-up.d:
total 72
-rwxr-xr-x 1 root root  817 Jul 20  2013 000resolvconf
-rwxr-xr-x 1 root root 4359 May 29  2016 00check-network-cable
-rwxr-xr-x 1 root root 4341 Oct  1  2014 00check-network-cable.dpkg-old
-rwxr-xr-x 1 root root 5521 Feb  6  2018 10check-duplicate-ip
-rwxr-xr-x 1 root root 3848 Feb  6  2016 10check-duplicate-ip6
-rwxr-xr-x 1 root root 4919 Feb  6  2018 20static-routes
-rwxr-xr-x 1 root root 4566 Apr 10  2014 30check-gateway
-rwxr-xr-x 1 root root  923 Apr 13  2015 avahi-autoipd
-rwxr-xr-x 1 root root  484 Mar  6  2013 avahi-daemon
-rwxr-xr-x 1 root root 1685 Sep 22  2014 ethtool
-rwxr-xr-x 1 root root 4937 Aug 22 17:39 mountnfs
-rwxr-xr-x 1 root root  385 Jul  4  2016 openvpn
lrwxrwxrwx 1 root root   32 Aug  8 15:59 wpasupplicant -> ../../wpa_supplicant/ifupdown.sh