Add SR_CONF keys to report the number of timebases and vdivs

Number of timebases is needed to properly interpret a frame's worth
of samples. Number of vertical divisions isn't, but may nevertheless
be interesting for a frontend that wants to reproduce the scope's
native display.

libsigrok.h

@@ -636,6 +636,12 @@ enum {
 	/** The device supports setting its sample interval, in ms. */
 	SR_CONF_SAMPLE_INTERVAL,
 
+	/** Number of timebases, as related to SR_CONF_TIMEBASE.  */
+	SR_CONF_NUM_TIMEBASE,
+
+	/** Number of vertical divisions, as related to SR_CONF_VDIV.  */
+	SR_CONF_NUM_VDIV,
+
 	/*--- Special stuff -------------------------------------------------*/
 
 	/** Scan options supported by the driver. */