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

libexec/bootpd/bootpgw/bootpgw.c

@@ -84,7 +84,7 @@ __FBSDID("$FreeBSD$");
  * Externals, forward declarations, and global variables
  */
 
-static void usage(void);
+static void usage(void) __dead2;
 static void handle_reply(void);
 static void handle_request(void);
 

libexec/bootpd/tools/bootpef/bootpef.c

@@ -78,7 +78,7 @@ SOFTWARE.
  */
 
 static void mktagfile(struct host *);
-static void usage(void);
+static void usage(void) __dead2;
 
 /*
  * General