More adjustments of sfileno/sdirno.

More adjustments of sfileno/sdirno.

* sdirno type changed back to int, as GCC does not like to use char
  for integer operations (array indexing).
* sizes adjusted to
   swap_filen:25
   swap_dirn:7
   This limits the cache size to
      files/cache_dir:  2^24  (16777216)
      cache_dirs     :  2^6-1 (63)
* a couple of asserts added to ensure the above limits.