util, types: parse unix_socket_permission strictly

Default "0o770" failed the base-8 parse and silently became 0700.
Invalid, unquoted or above-0777 values now fail config load.

Fixes #3529

config-example.yaml

@@ -361,6 +361,7 @@ dns:
 # Unix socket used for the CLI to connect without authentication
 # Note: for production you will want to set this to something like:
 unix_socket: /var/run/headscale/headscale.sock
+# Octal, quoted: YAML reads an unquoted 0770 as the number 504.
 unix_socket_permission: "0770"
 
 # OpenID Connect