Fix dhcpcd failing on systemd-resolved stub

dhcpcd fails to update /etc/resolv.conf if is a dangling symlink, so remove it
if it is.

This happens on Arch Linux when systemd-resolved is enabled by symlinking
/etc/resolv.conf to /run/systemd/resolve/stub-resolv.conf. The symlink is copied
into the supermin appliance, but the target file is not and this causes dhcpcd to
fail when it tries to update /etc/resolv.conf.

The handling of /etc/resolv.conf could be improved in dhcpcd but it's not their
job to decide when to remove the symlink. We do have the authority to decide
whether or not to use systemd-resolved or resolvconf in the appliance, so we can
remove it.

appliance/init

@@ -115,6 +115,10 @@ if test "$guestfs_network" = 1; then
     # Two workarounds for Ubuntu:
     touch /etc/fstab
     rm -f /etc/dhcp/dhclient-enter-hooks.d/resolved
+    # Prevent dhcpcd from failing when /etc/resolv.conf is a dangling symlink.
+    if [ -L "/etc/resolv.conf" ] && [ ! -e "/etc/resolv.conf" ]; then
+        rm -f /etc/resolv.conf
+    fi
     if dhclient --version >/dev/null 2>&1; then
         dhclient $iface
     elif dhcpcd $iface; then