xz: Add a default soft memory usage limit for --threads=0.

This is a soft limit in sense that it only affects the number of
threads. It never makes xz fail and it never makes xz change
settings that would affect the compressed output.

The idea is to make -T0 have more reasonable behavior when
the system has very many cores or when a memory-hungry
compression options are used. This also helps with 32-bit xz,
preventing it from running out of address space.

The downside of this commit is that now the number of threads
might become too low compared to what the user expected. I
hope this to be an acceptable compromise as the old behavior
has been a source of well-argued complaints for a long time.

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