Raise config syntax error for N0CALL/NOCALL

Check to see if the user tries to start APRX with one of the two
placeholder callsigns and error out if they do instead of trying
to DDoS the APRS-IS servers

Closes #64

Author: PhirePhly

config.c

@@ -22,6 +22,10 @@ char *config_SKIPSPACE(char *Y) {
 	return Y;
 }
 
+// Validate the callsign configured for APRX
+// Ensure that it meets the requirements for an APRS callsign
+// and if it is also a valid AX.25 callsign if strict is true.
+//
 // return 0 for failures, 1 for OK.
 int validate_callsign_input(char *callsign, int strict) {
 	int i = strlen(callsign);
@@ -82,6 +86,15 @@ int validate_callsign_input(char *callsign, int strict) {
 		callsign[i - 2] = 0;
 		/* If tailed with "-0" SSID, chop it off.. */
 	}
+
+	// Check for invalid default callsigns
+	if (strncmp(callsign, "N0CALL", 6) == 0) {
+		return 0;
+	}
+	if (strncmp(callsign, "NOCALL", 6) == 0) {
+		return 0;
+	}
+
 	return 1;
 }