Add lzma_physmem().

Add lzma_physmem().

I had hoped to keep liblzma as purely a compression
library as possible (e.g. file I/O will go into
a different library), but it seems that applications
linking agaisnt liblzma need some way to determine
the memory usage limit, and knowing the amount of RAM
is one reasonable way to help making such decisions.

Thanks to Jonathan Nieder for the original patch.