Improved the memory limitter:

Improved the memory limitter:
  - Added lzma_memlimit_max() and lzma_memlimit_reached()
    API functions.
  - Added simple estimation of malloc()'s memory usage
    overhead.
  - Fixed integer overflow detection in lzma_memlimit_alloc().
  - Made some white space cleanups and added more comments.

The description of lzma_memlimit_max() in memlimit.h is bad
and should be improved.