Specify C stack size when creating schedulers

[brson]

We appear to have 1MB C stacks right now, which is not enough for some applications, like spidermonkey. When creating a new scheduler we should be able to specify the size of the C stack.

[msullivan]

This is somewhat related to other stack considerations, like #5768. And probably to the scheduler rewrite also.

[thestinger]

This is still an open issue.

[brson]

This was never nominated. Removing milestone.

[brson]

Obsolete with current stack plans.