liblzma: Add the internal function lzma_alloc_zero().
liblzma: Use lzma_alloc_zero() in LZ encoder initialization.

This avoids a memzero() call for a newly-allocated memory,
which can be expensive when encoding small streams with
an over-sized dictionary.

To avoid using lzma_alloc_zero() for memory that doesn't
need to be zeroed, lzma_mf.son is now allocated separately,
which requires handling it separately in normalize() too.

Thanks to Vincenzo Innocente for reporting the problem.
Update THANKS.