Custom UID and GID for subprocesses and files written (#116) Fix #116

- Environment variables `UID` and `GID`, both defaulting to `1000`
- All subprocesses (openvpn, tinyproxy, etc.) run using the UID and GID given
- All files are written with an ownership for the UID and GID given
- Port forwarded file has also ownership for UID, GID and read permission only

internal/params/params.go

@@ -28,6 +28,10 @@ type ParamsReader interface {
 	GetDNSOverTLSPrivateAddresses() (privateAddresses []string)
 	GetDNSOverTLSIPv6() (ipv6 bool, err error)
 
+	// IDs
+	GetUID() (uid int, err error)
+	GetGID() (gid int, err error)
+
 	// Firewall getters
 	GetExtraSubnets() (extraSubnets []net.IPNet, err error)