Added configuration support for:

Added configuration support for:
1) 4KB, 8KB, and 16KB stacks
2) separate IRQSTACKS
3) stack overflow warnings at a user-specified size

The original FC code only supports 4KB kernel stacks w/ a separate,
per-cpu, IRQ stack and the stack overflow warning was printed whenever
the kernel approached 1/8 of the current stack size.

The new support permits one to select different stack sizes and
separate IRQ stack using configuration options.  The default right
now is to use a 8KB kernel stack with a separate IRQ stack.