build: make "./build.sh install" reusable

[oidq]

• prevent the script from blindly overwriting the current configuration
• force maddyctl symbolic linking
• fix indentation in build.sh

Few days ago I learned the hard way, that running ./build.sh install on existing installation of maddy blindly overwrites the configuration. This commit alters the bahevior of build.sh to not blindly copy over existing maddy.conf and force the creation of the maddyctl symlink so that the script can be used to update current installation.