Improve environment variable from file support

[aentwist]

The environment variable from file support is currently very fragmented on the PHP side.

https://github.com/nextcloud/docker/blob/31c59aea24f8516f862ca136766168016e840593/.config/autoconfig.php#L9-L37

This is extremely fragile, as it is all or nothing with the _FILE environment variables - and to complicate things, one of them is non-_FILE. Note that some environment variables such as these must also be supported in the entrypoint script.

https://github.com/nextcloud/docker/blob/31c59aea24f8516f862ca136766168016e840593/.config/smtp.config.php#L15-L21

---

These should be refactored to use a common function f(string) -> string that returns the environment variable value given its name and an optional default (no matter whether it is from file or not). There is already a somewhat parallel idea implemented in the entrypoint script -

https://github.com/nextcloud/docker/blob/31c59aea24f8516f862ca136766168016e840593/docker-entrypoint.sh#L22-L26

---

Related to https://github.com/nextcloud/docker/issues/1938