Implement Stack Usage Monotoring

[gerth2]

Each process in the RTOS uses a certain amt. of stack space, which varies at runtime with the depth of functions called. Motoring the peak stack usage is crucial to determining how many resources to allocate to each process, and predicting runtime errors from stack overflow.

Add functions and processes to do the following:

1. After processes are created but before they are started, "Paint" the allocated stack space for the process with a deterministic pattern of bits
2. At various points during runtime, walk each processes stack from top to bottom to determine the amount of "Undisturbed" stack left (assume that when a process actually uses stack space, it overwrites the deterministic pattern from step 1). The % stack usage is 100*(1-undisturbed_stack/total_stack)

Be sure to check for interactions with the RTOS's existing stack overflow checking functionality (especially method 2):

http://www.freertos.org/Stacks-and-stack-overflow-checking.html

This % usage calculation could likely just be an extension of method 2. Design is up to developer.

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