Arguments to ctype functions are required to be either EOF or representable

as unsigned char.  This means that passing in a bare char is incorrect on
systems where char is signed.  Sprinkle some appropriate casts to prevent
this.

[originally from svn r8406]

cmdgen.c

@@ -119,7 +119,7 @@ void sk_cleanup(void)
 void showversion(void)
 {
     char *verstr = dupstr(ver);
-    verstr[0] = tolower(verstr[0]);
+    verstr[0] = tolower((unsigned char)verstr[0]);
     printf("PuTTYgen %s\n", verstr);
     sfree(verstr);
 }