chore!(amneziawg): refactor to be separate from wireguard

- amneziawg is now a VPN protocol and no longer a Wireguard implementation
- Use it with VPN_TYPE=amneziawg
- document AMNEZIAWG_* options in Dockerfile
- document amneziawg support in readme
- separate amneziawg settings and code from wireguard
- re-use code from wireguard whenever possible

internal/cleanup/cleanup.go

@@ -0,0 +1,51 @@
+package cleanup
+
+import "sort"
+
+type Cleanups []cleanup
+
+type cleanup struct {
+	operation  string
+	orderIndex uint
+	cleanup    func() error
+	done       bool
+}
+
+// Add adds a cleanup function to the list of cleanups, with a description of the
+// operation being cleaned up, and an order index that determines the order in which
+// the cleanup functions are run. The lower the order index, the earlier the cleanup
+// function is run.
+func (c *Cleanups) Add(operation string, orderIndex uint,
+	cleanupFunc func() error,
+) {
+	closer := cleanup{
+		operation:  operation,
+		orderIndex: orderIndex,
+		cleanup:    cleanupFunc,
+	}
+	*c = append(*c, closer)
+}
+
+// Cleanup runs the cleanup functions in the order of their orderIndex,
+// and logs any error that occurs during cleanup.
+// It can also be re-called in case a cleanup fails, and already cleaned up
+// functions will not be re-run.
+func (c *Cleanups) Cleanup(logger Logger) {
+	closers := *c
+
+	sort.Slice(closers, func(i, j int) bool {
+		return closers[i].orderIndex < closers[j].orderIndex
+	})
+
+	for i, closer := range closers {
+		if closer.done {
+			continue
+		}
+		closers[i].done = true
+		logger.Debug(closer.operation + "...")
+		err := closer.cleanup()
+		if err != nil {
+			logger.Error("failed " + closer.operation + ": " + err.Error())
+		}
+	}
+}