xz: Add initial support for threaded decompression.

If threading support is enabled at build time, this will
use lzma_stream_decoder_mt() even for single-threaded mode.
With memlimit_threading=0 the behavior should be identical.

This needs some work like adding --memlimit-threading=LIMIT.

The original patch from Sebastian Andrzej Siewior included
a method to get currently available RAM on Linux. It might
be one way to go but as it is Linux-only, the available-RAM
approach needs work for portability or using a fallback method
on other OSes.

The man page wasn't updated yet.