Add prototypes for main() so that these programs compile with -Werror

(which somehow now seems to be the default for compiling -current).
This error popped up while doing a PicoBSD cross-compile on a 4.3-ish system,
it may well be that there are other apps which have similar problems,
but I did not spot them as they are not included in my picobsd config.

Whether adding prototypes for main() is the correct solution or not
I have no idea, a request to -current on the matter went basically
unanswered. Those who have better ideas are welcome to back this out
and replace it with the correct fix.

bin/rm/rm.c

@@ -69,6 +69,7 @@ void	rm_file __P((char **));
 void	rm_overwrite __P((char *, struct stat *));
 void	rm_tree __P((char **));
 void	usage __P((void));
+int	main __P((int argc, char *argv[]));
 
 /*
  * rm --

usr.sbin/chown/chown.c

@@ -64,6 +64,7 @@ void	a_uid __P((const char *));
 void	chownerr __P((const char *));
 u_long	id __P((const char *, const char *));
 void	usage __P((void));
+int	main __P((int argc, char *argv[]));
 
 uid_t uid;
 gid_t gid;

usr.sbin/dev_mkdb/dev_mkdb.c

@@ -61,6 +61,7 @@ static const char rcsid[] =
 #include <unistd.h>
 
 static void	usage __P((void));
+int 		main __P((int argc, char *argv[]));
 
 int
 main(argc, argv)