Implementation of name mangling algorithm.

This has to ensure that file names are unique among all files in an iso 
directory. Current implementation can be improved by using a hash table 
to keep the names, instead of iterate over all children. An alternative 
method, keeping children sorted all the time, thus allowing binary 
search, has been considered but discarded.                          

Current implementation support up to 9,999,999 equal files per 
directory.

src/util.c

@@ -27,6 +27,15 @@ int round_up(int n, int mul)
     return div_up(n, mul) * mul;
 }
 
+int int_pow(int base, int power)
+{
+    int result = 1;
+    while (--power >= 0) {
+        result *= base;
+    }
+    return result;
+}
+
 /**
  * Convert a str in a specified codeset to WCHAR_T. 
  * The result must be free() when no more needed