Fix default config permissions (#1253)

Currently, all init scripts, except for systemd, will generate a config
file with default permissions, which is usually `rw-r--r--`.
This is bad, because the config contains a private key.

The systemd service does `chmod 640` after creating the config, which is
much better than just leaving it readable for everyone forever, but
there is still a slight chance that some malicious program might steal
the private key during the time window between key creation and chmod.

For this reason, in this pull request I use `umask 037`, so the config
won't have read permission for others in the first place.

Note that I have only tested openrc and systemd services.

Also, I'm not sure what to do with the contrib/msi/build-msi.sh script,
which creates a bat file that generates a config. I don't know anything
about file permissions on windows, however, it seems that the bat file
generates the config into a user's personal directory, so maybe it's
already somewhat fine.

contrib/docker/entrypoint.sh

@@ -6,7 +6,7 @@ CONF_DIR="/etc/yggdrasil-network"
 
 if [ ! -f "$CONF_DIR/config.conf" ]; then
   echo "generate $CONF_DIR/config.conf"
-  yggdrasil --genconf > "$CONF_DIR/config.conf"
+  (umask 037 && yggdrasil --genconf > "$CONF_DIR/config.conf")
 fi
 
 if [ -n "$ALLOW_IPV6_FORWARDING" ]; then