Declare some function arguments as const.

Some functions, mostly in fight.c, declared variables as pointers to
const char but passed them to functions that took pointers to ordinary
char.  The strings did not actually get modified, so adding 'const' to
the function definitions removed the warnings.

rogue4/misc.c

@@ -345,7 +345,7 @@ aggravate(void)
  *	"an".
  */
 char *
-vowelstr(char *str)
+vowelstr(const char *str)
 {
     switch (*str)
     {