Protect terminal dump from vagaries of C libray implementation of isalpha() etc

Some C libraries assign true to isalpha(0xff), isdigit(0xff) or
ispunct(0xff), which means that the Operating System terminal sees a
character 0xff which it may not have a useful display character for.

This commit only outputs printable ASCII characters for an AVRDUDE
terminal dump reducing the risk of the OS terminal not being able
to print the character properly.

src/term.c

@@ -198,13 +198,9 @@ static int chardump_line(char * buffer, unsigned char * p, int n, int pad)
   n = n < 1? 1: n > sizeof b? sizeof b: n;
 
   memcpy(b, p, n);
-  for (int i = 0; i < n; i++) {
-    buffer[i] = '.';
-    if (isalpha(b[i]) || isdigit(b[i]) || ispunct(b[i]))
-      buffer[i] = b[i];
-    else if (isspace(b[i]))
-      buffer[i] = ' ';
-  }
+  for (int i = 0; i < n; i++)
+    buffer[i] = isascii(b[i]) && isspace(b[i])? ' ':
+      isascii(b[i]) && isgraph(b[i])? b[i]: '.';
 
   for (i = n; i < pad; i++)
     buffer[i] = ' ';