xz: Changes to --memlimit-compress and --no-adjust.

In single-threaded mode, --memlimit-compress can make xz scale down
the LZMA2 dictionary size to meet the memory usage limit. This
obviously affects the compressed output. However, if xz was in
threaded mode, --memlimit-compress could make xz reduce the number
of threads but it wouldn't make xz switch from multithreaded mode
to single-threaded mode or scale down the LZMA2 dictionary size.
This seemed illogical and there was even a "FIXME?" about it.

Now --memlimit-compress can make xz switch to single-threaded
mode if one thread in multithreaded mode uses too much memory.
If memory usage is still too high, then the LZMA2 dictionary
size can be scaled down too.

The option --no-adjust was also changed so that it no longer
prevents xz from scaling down the number of threads as that
doesn't affect compressed output (only performance). After
this commit --no-adjust only prevents adjustments that affect
compressed output, that is, with --no-adjust xz won't switch
from multithreaded mode to single-threaded mode and won't
scale down the LZMA2 dictionary size.

The man page wasn't updated yet.

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