eliminate some magic numbers in tetris

[Theo Buehler]

Trivial turd polishing, inspired by something similar done in Dragonfly
but using the more familiar idiom for a for loop.

Index: games/tetris/tetris.c
===================================================================
RCS file: /var/cvs/src/games/tetris/tetris.c,v
retrieving revision 1.32
diff -u -p -r1.32 tetris.c
--- games/tetris/tetris.c       13 Aug 2017 02:12:16 -0000      1.32
+++ games/tetris/tetris.c       11 Apr 2018 18:20:35 -0000
@@ -52,6 +52,8 @@
 #include "screen.h"
 #include "tetris.h"
 
+#define NUMKEYS 6
+
 cell   board[B_SIZE];
 int    Rows, Cols;
 const struct shape *curshape;
@@ -142,7 +144,6 @@ randshape(void)
                tmp = &shapes[classic? tmp->rotc : tmp->rot];
        return (tmp);
 }
-       
 
 int
 main(int argc, char *argv[])
@@ -150,7 +151,7 @@ main(int argc, char *argv[])
        int pos, c;
        char *keys;
        int level = 2;
-       char key_write[6][10];
+       char key_write[NUMKEYS][10];
        const char *errstr;
        int ch, i, j;
 
@@ -171,7 +172,7 @@ main(int argc, char *argv[])
                        classic = 1;
                        break;
                case 'k':
-                       if (strlen(keys = optarg) != 6)
+                       if (strlen(keys = optarg) != NUMKEYS)
                                usage();
                        break;
                case 'l':
@@ -199,8 +200,8 @@ main(int argc, char *argv[])
 
        fallrate = 1000000000L / level;
 
-       for (i = 0; i <= 5; i++) {
-               for (j = i+1; j <= 5; j++) {
+       for (i = 0; i < NUMKEYS; i++) {
+               for (j = i+1; j < NUMKEYS; j++) {
                        if (keys[i] == keys[j])
                                errx(1, "duplicate command keys specified.");
                }