updates-proxy-setup: support setting proxy for apt (#887)

vm-systemd/misc-post.sh

@@ -1,9 +1,19 @@
 #!/bin/sh
 
 if [ -f /var/run/qubes-service/yum-proxy-setup -o -f /var/run/qubes-service/updates-proxy-setup ]; then
-    echo proxy=http://10.137.255.254:8082/ > /etc/yum.conf.d/qubes-proxy.conf
+    if [ -d /etc/apt/apt.conf.d ]; then
+        echo 'Acquire::http::Proxy "http://10.137.255.254:8082/;"' >> /etc/apt/apt.conf.d/01qubes-proxy
+    fi
+    if [ -d /etc/yum.conf.d ]; then
+        echo proxy=http://10.137.255.254:8082/ > /etc/yum.conf.d/qubes-proxy.conf
+    fi
 else
-    echo > /etc/yum.conf.d/qubes-proxy.conf
+    if [ -d /etc/apt/apt.conf.d ]; then
+        rm -f /etc/apt/apt.conf.d/01qubes-proxy
+    fi
+    if [ -d /etc/yum.conf.d ]; then
+        echo > /etc/yum.conf.d/qubes-proxy.conf
+    fi
 fi
 
 # Set IP address again (besides action in udev rules); this is needed by