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>

lib.h

@@ -47,6 +47,13 @@
 
 #include <stdio.h>
 
+#ifdef DEBUG
+#define pr_debug(fmt, args...) printf(fmt, ##args)
+#else
+__attribute__((format (printf, 1, 2)))
+static inline int pr_debug(const char* fmt, ...) {return 0;}
+#endif
+
 /* buffer sizes for CAN frame string representations */
 
 #define CL_ID (sizeof("12345678##1"))