liblzma: Use lzma_memcmplen() in the BT3 match finder.

I had missed this when writing the commit
5db75054e900fa06ef5ade5f2c21dffdd5d16141.

Thanks to Jun I Jin.
liblzma: Add support for lzma_block.ignore_check.

Note that this slightly changes how lzma_block_header_decode()
has been documented. Earlier it said that the .version is set
to the lowest required value, but now it says that the .version
field is kept unchanged if possible. In practice this doesn't
affect any old code, because before this commit the only
possible .version was 0.
liblzma: Add support for LZMA_IGNORE_CHECK.
xz: Add --ignore-check.