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/systemd/yggdrasil-default-config.service.debian

@@ -8,6 +8,7 @@ After=local-fs.target
 [Service]
 Type=oneshot
 Group=yggdrasil
+UMask=037
 ExecStartPre=/usr/bin/mkdir -p /etc/yggdrasil
 ExecStart=/usr/bin/yggdrasil -genconf > /etc/yggdrasil/yggdrasil.conf
 ExecStartPost=/usr/bin/chmod -R 0640 /etc/yggdrasil