Added support for flush marker, which will be in files

Added support for flush marker, which will be in files
that use LZMA_SYNC_FLUSH with encoder (not implemented
yet). This is a new feature in the raw LZMA format,
which isn't supported by old decoders. This shouldn't
be a problem in practice, since lzma_alone_encoder()
will not allow LZMA_SYNC_FLUSH, and thus not allow
creating files on decodable with old decoders.

Made lzma_decoder.c to require tab width of 4 characters
if one wants to fit the code in 80 columns. This makes
the code easier to read.