chore(settings): refactor settings processing (#756)

- Better settings tree structure logged using `qdm12/gotree`
- Read settings from environment variables, then files, then secret files
- Settings methods to default them, merge them and override them
- `DNS_PLAINTEXT_ADDRESS` default changed to `127.0.0.1` to use DoT. Warning added if set to something else.
- `HTTPPROXY_LISTENING_ADDRESS` instead of `HTTPPROXY_PORT` (with retro-compatibility)

internal/configuration/sources/files/system.go

@@ -0,0 +1,10 @@
+package files
+
+import (
+	"github.com/qdm12/gluetun/internal/configuration/settings"
+)
+
+func (r *Reader) readSystem() (system settings.System, err error) {
+	// TODO timezone from /etc/localtime
+	return system, nil
+}