cli: Introduce a logfile command to log errors to a file

The evolution would be to introduce libosmocore and start using
the logging framework. But even then we can map this option to
the file target.

Fixes: SYS#263
diff --git a/ggsn/cmdline.ggo b/ggsn/cmdline.ggo
index e7e0f66..5bb05dc 100644
--- a/ggsn/cmdline.ggo
+++ b/ggsn/cmdline.ggo
@@ -31,5 +31,5 @@
 
 option  "apn"         a "Access point name"             string default="internet" no
 option  "qos"         q "Requested quality of service"  int    default="0x0b921f" no
-
+option  "logfile"     - "Logfile for errors"            string no