Mark usage function as __dead2 in programs where it does not return

In most cases, usage does not return, so mark them as __dead2. For the
cases where they do return, they have not been marked __dead2.

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/735

tests/sys/cddl/zfs/bin/file_write.c

@@ -38,7 +38,7 @@ static unsigned char bigbuffer[BIGBUFFERSIZE];
  * See header file for defaults.
  */
 
-static void usage(void);
+static void usage(void) __dead2;
 static char *execname;
 
 int

tests/sys/netinet/libalias/perf.c

@@ -38,7 +38,7 @@
 #include "util.h"
 #include <alias.h>
 
-static void usage(void);
+static void usage(void) __dead2;
 
 #define	timevalcmp(tv, uv, cmp)			\
 	(((tv).tv_sec == (uv).tv_sec)		\