Add more _printf_'s for format-nonliterals

Clang is a bit more strict wrt format-nonliterals:
http://clang.llvm.org/docs/LanguageExtensions.html#format-string-checking

Adding these extra printf attributes also makes gcc able to find more
problems. E.g. this patch uncovers a format issue in udev-builtin-path_id.c

Some parts looked intetional about breaking the format-nonliteral check.
I added some supression for warnings there.

Signed-off-by: Anthony G. Basile <blueness@gentoo.org>

src/accelerometer/accelerometer.c

@@ -71,6 +71,7 @@
 
 static int debug = 0;
 
+_printf_(6,0)
 static void log_fn(struct udev *udev, int priority,
                    const char *file, int line, const char *fn,
                    const char *format, va_list args)

src/ata_id/ata_id.c

@@ -405,6 +405,7 @@ out:
         return ret;
 }
 
+_printf_(6,0)
 static void log_fn(struct udev *udev, int priority,
                    const char *file, int line, const char *fn,
                    const char *format, va_list args)

src/cdrom_id/cdrom_id.c

@@ -43,6 +43,7 @@
 
 static bool debug;
 
+_printf_(6,0)
 static void log_fn(struct udev *udev, int priority,
                    const char *file, int line, const char *fn,
                    const char *format, va_list args)

src/libudev/libudev.c

@@ -65,6 +65,7 @@ void udev_log(struct udev *udev,
         va_end(args);
 }
 
+_printf_(6,0)
 static void log_stderr(struct udev *udev,
                        int priority, const char *file, int line, const char *fn,
                        const char *format, va_list args)

src/scsi_id/scsi_id.c

@@ -67,6 +67,7 @@ static char model_enc_str[256];
 static char revision_str[16];
 static char type_str[16];
 
+_printf_(6,0)
 static void log_fn(struct udev *udev, int priority,
                    const char *file, int line, const char *fn,
                    const char *format, va_list args)

src/udev/udev-builtin-kmod.c

@@ -73,6 +73,7 @@ static int load_module(struct udev *udev, const char *alias)
 #endif
 }
 
+_printf_(6,0)
 static void udev_kmod_log(void *data, int priority, const char *file, int line,
                           const char *fn, const char *format, va_list args)
 {

src/udev/udev-builtin-path_id.c

@@ -32,6 +32,7 @@
 
 #include "udev.h"
 
+_printf_(2,3)
 static int path_prepend(char **path, const char *fmt, ...)
 {
         va_list va;

test/test-libudev.c

@@ -33,6 +33,7 @@
 
 #define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
 
+_printf_(6,0)
 static void log_fn(struct udev *udev,
                    int priority, const char *file, int line, const char *fn,
                    const char *format, va_list args)