xz: Make --block-list and --block-size work together in single-threaded.

Previously, --block-list and --block-size only worked together
in threaded mode. Boundaries are specified by --block-list, but
--block-size specifies the maximum size for a Block. Now this
works in single-threaded mode too.

Thanks to James M Leddy for the original patch.
Update THANKS.
xz: Update the man page about --block-size and --block-list.
liblzma: Document the need for block->check for lzma_block_header_decode().

Thanks to Tomer Chachamu.
Update THANKS.