Fix mkdtemp() random signedness

time_t return of time() may be signed 32-bit and in that case
probably will roll over in the year 2038 and yield a negative
value; signedness was propagated in the XOR operation to the
'value' and then 'v' variables. The 'v % 62' operation then would
had resulted in a negative value and LETTER[v%62] would had
accessed an arbitrary data location.

The same could had happened if the static long 'value' variable
after a very long run time contained a sufficiently large value to
which the time^pid value added resulted in a wrap / roll-over to a
negative value.

Using unsigned long types for 'value' and 'v' and casting time_t
to unsigned long cures all this.

mkdtemp.c

···
 char *mkdtemp (char *tmpl)
 {
   static const char LETTERS[] = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
-  static long       value = 0;
-  long              v;
+  static unsigned long value = 0;
+  unsigned long     v;
   int               len;
   int               i, j;
 
···
     return NULL;
   }
 
-  value += ((long) time (NULL)) ^ getpid ();
+  value += ((unsigned long) time (NULL)) ^ getpid ();
 
   for (i = 0; i < 7 ; ++i, value += 7777)
   {