lib: add pr_debug() macro

Add the pr_debug() macro so that:

  #ifdef DEBUG
  	printf("foo");
  #endif

can be replaced by:

  	pr_debug("foo");

Apply the pr_debug() macro wherever relevant.

Currently, there is no consensus whether debug messages should be
printed on stdout or stderr. Most of the modules: canbusload.c,
candump.c and canlogserver.c use stdout but
mcp251xfd/mcp251xfd-dev-coredump.c uses stderr. Harmonize the behavior
by following the major trend and make
mcp251xfd/mcp251xfd-dev-coredump.c also output to stdout.

slcanpty.c does a #define DEBUG, meaning that debug is always turned
on for this file. Remove this and make debug an option like every
other files.

Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Acked-by: Oliver Hartkopp <socketcan@hartkopp.net>
Link: https://lore.kernel.org/all/20221114163848.3398-5-mailhol.vincent@wanadoo.fr
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

canlogserver.c

@@ -145,9 +145,7 @@ int idx2dindex(int ifidx, int socket)
 
 	strcpy(devname[i], ifr.ifr_name);
 
-#ifdef DEBUG
-	printf("new index %d (%s)\n", i, devname[i]);
-#endif
+	pr_debug("new index %d (%s)\n", i, devname[i]);
 
 	return i;
 }
@@ -310,11 +308,9 @@ int main(int argc, char **argv)
 
 	for (i=0; i<currmax; i++) {
 
-#ifdef DEBUG
-		printf("open %d '%s' m%08X v%08X i%d e%d.\n",
-		       i, argv[optind+i], mask[i], value[i],
-		       inv_filter[i], err_mask[i]);
-#endif
+		pr_debug("open %d '%s' m%08X v%08X i%d e%d.\n",
+		      i, argv[optind+i], mask[i], value[i],
+		      inv_filter[i], err_mask[i]);
 
 		if ((s[i] = socket(PF_CAN, SOCK_RAW, CAN_RAW)) < 0) {
 			perror("socket");