[network-proxy-setup] Permit !CONFIG_MODuLES

* Check whether sysctl is accessible
* Check whether a key which exists when CONFIG_MODULES=y is not accessible

If true, CONFIG_MODULES=n, so ignore modprobe failure.
If false, fail.

vm-systemd/network-proxy-setup.sh

@@ -3,10 +3,17 @@
 # Setup gateway for all the VMs this netVM is serviceing...
 network=$(qubesdb-read /qubes-netvm-network 2>/dev/null)
 if [ "x$network" != "x" ]; then
+
+    if [ -e /proc/sys/kernel ] && ! [ -e /proc/sys/kernel/modules_disabled ]; then
+      readonly modprobe_fail_cmd='true'
+    else
+      readonly modprobe_fail_cmd='false'
+    fi
+
     gateway=$(qubesdb-read /qubes-netvm-gateway)
     netmask=$(qubesdb-read /qubes-netvm-netmask)
     secondary_dns=$(qubesdb-read /qubes-netvm-secondary-dns)
-    modprobe netbk 2> /dev/null || modprobe xen-netback
+    modprobe netbk 2> /dev/null || modprobe xen-netback || "${modprobe_fail_cmd}"
     echo "NS1=$gateway" > /var/run/qubes/qubes-ns
     echo "NS2=$secondary_dns" >> /var/run/qubes/qubes-ns
     /usr/lib/qubes/qubes-setup-dnat-to-ns