Add Trimble RT17 support

This pull is for minor modifications to add Trimble RT17 raw stream and file support to RTKLIB. Please see the top of the included rt17.c source file for details. I have also included minor documentation changes.

Two issues with this code, for which I need your help, may interest you enough to comment on or resolve them.

1. In the file "rt17.c", function "decode_type_17", the value assigned to the "l2_code" variable is questionable.

2. In the file "convrnx.c" function "set_obstype", the initialization of the array "codes_rt17" is questionable.

Please take a look at these and see what you can do to resolve them or help me to resolve them. Once resolved, please push the code into RTKLIB.

Thank you,