[sgen] Option for an allowance limit.

[sgen] Option for an allowance limit.

The metric for deciding when to trigger a major collection assumes
that there's unlimited memory, so it does, especially since it's
probably buggy and certainly not very smart, sometimes allow too much
allocation to happen.  This new option is a way to tell SGen where to
draw the line.  Note that memory will still be allocated, it's just
that once the limit is overstepped, major collections happen more
frequently to keep memory usage down.