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>

canbusload.c

@@ -61,6 +61,7 @@
 #include <linux/can.h>
 #include <linux/can/raw.h>
 
+#include "lib.h"
 #include "terminal.h"
 #include "canframelen.h"
 
@@ -310,9 +311,7 @@ int main(int argc, char **argv)
 			return 1;
 		}
 
-#ifdef DEBUG
-		printf("open %d '%s'.\n", i, ptr);
-#endif
+		pr_debug("open %d '%s'.\n", i, ptr);
 
 		s[i] = socket(PF_CAN, SOCK_RAW, CAN_RAW);
 		if (s[i] < 0) {
@@ -358,10 +357,8 @@ int main(int argc, char **argv)
 		if (nbytes > max_bitrate_len)
 			max_bitrate_len = nbytes; /* for nice printing */
 
+		pr_debug("using interface name '%s'.\n", ifr.ifr_name);
 
-#ifdef DEBUG
-		printf("using interface name '%s'.\n", ifr.ifr_name);
-#endif
 		/* try to switch the socket into CAN FD mode */
 		const int canfd_on = 1;
 		setsockopt(s[i], SOL_CAN_RAW, CAN_RAW_FD_FRAMES, &canfd_on, sizeof(canfd_on));