Add support for PTT-related commands to rigctl_server

**Feature description**
The "t" command is necessary in many software applications to validate the rigctld. If SDR++ returns invalid values, the software will consider the radio invalid.
```
T, set_ptt 'PTT'
Set 'PTT'.
PTT is a value: ‘0’ (RX), ‘1’ (TX), ‘2’ (TX mic), or ‘3’ (TX data).

t, get_ptt
Get 'PTT' status.
Returns PTT as a value in set_ptt above.
```