Fix rust-lang#2229 detect available memory

This doesn't implement streaming IO for low memory situations - we still
have the situation that low footprint situations will fail to install,
but while it is the case that rustc's memory footprint is lower than our
unpack footprint this is probably not urgent to fix, though I will get
around to it.

Being less aggressive about unpack buffer size though should reduce the
number of support tickets from folk in these cases, I hope.

We may end up getting tickets from folk with broken ulimit syscalls
though, who knows.