Patch: EPC capability flag support

Hi, as a part of our project at university, we implemented extraction of EPC Capability flag. This flag is a bit sent by mobile phone to indicate support of EPC (or LTE). This bit is extracted during processing of Attach request to log this information. An function to check the the of this flag for given sgsn context is also created to make it reusable. For the extraction of the EPC Capability field we used the location described in TS 24.008 v10.15.0 section 10.5.5.12. This can be interesting information for any developers experimenting with devices supporting 4G, so I think I would be a good idea to make it available in the original repository. The patch for this change is in the attachment. Best regards, Tomas Movay