Make OAC-cmdgw to process the begin line

And check whether it contains any error.
Add `WRITE_OFFSET` command to OACSP and cmd-gw

Add a new command `WRITE_OFFSET` to both OACSP and Command Gateway.
This command may be used to write a value to a FSUIPC offset.
Implement LVAR observe in OACSP and CMD GW

New operations have been added to OACSP in order to observe LVARs.
The corresponding code has been added to CMD GW to process such
observe requests.