Few problems building from v11 source

[arggh]

While building tang from source (...since apt install installed a buggy version 7) and installing it on Ubuntu 20.04, I bumped into some issues:

• The systemd service expects a tang user to be present on the system, which wasn't documented anywhere in the build instructions.
• Tang fails with Error calling stat() on path: /usr/local/var/db/tang: Permission denied. After adding o+rx permissions for each folder in the entire path I could start tang.