[mc] Robust handling of Forge, Bukkit, and Spigot version selection

* refactor server.properties sed ops into a utility function
* also switch to jq instead of jsawk
* simplify to BUILD_FROM_SOURCE from BUILD_SPIGOT_FROM_SOURCE
Fixes #70

minecraft-server/start.sh

@@ -11,4 +11,9 @@ while lsof -- /start-minecraft; do
   echo -n "."
   sleep 1
 done
-exec /start-minecraft
+
+mkdir /home/minecraft
+chown minecraft: /home/minecraft
+
+echo "Switching to user 'minecraft'"
+exec sudo -E -u minecraft /start-minecraft