Tweak NULL-pointer checks

Use !p and p when comparing pointers as opposed to explicit
checks against NULL.  This is generally easier to read.

cksum.c

@@ -114,7 +114,7 @@ cksum(FILE *fp, const char *s)
 		ck = (ck << 8) ^ crctab[(ck >> 24) ^ (i & 0xFF)];
 
 	printf("%"PRIu32" %lu", ~ck, (unsigned long)len);
-	if (s != NULL)
+	if (s)
 		printf(" %s", s);
 	putchar('\n');
 }