add die() to log an error message and exit with a single call

Almost all occurences of err() are either immediately followed by
exit(EXIT_FAILURE), usage(argv[0]) (which itself then calls
exit(EXIT_FAILURE), or that is what's done immediately after returning
from the function that calls err(). Modify the errfn macro so that its
instantiations can include exit(EXIT_FAILURE) at the end, and use that
to create a new function die() that will log an error and then
exit.

Signed-off-by: Laine Stump <laine@redhat.com>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

log.h

@@ -10,6 +10,7 @@
 #define LOGFILE_CUT_RATIO		30	/* When full, cut ~30% size */
 #define LOGFILE_SIZE_MIN		(5UL * MAX(BUFSIZ, PAGE_SIZE))
 
+void die(const char *format, ...);
 void err(const char *format, ...);
 void warn(const char *format, ...);
 void info(const char *format, ...);