Some environments in which execvPe may be called have a limited amount of stack available. Currently, it avoidably allocates a segment on the stack large enough to hold PATH so that it may be mutated and use strsep() for easy parsing. This logic is now rewritten to just operate on the immutable string passed in and do the necessary math to extract individual paths, since it will be copying out those segments to another buffer anyways and piecing them together with the name for a full path.
Additional size is also needed for the stack in posix_spawnp(), because it may need to push all of argv to the stack and rebuild the command with sh in front of it. We'll make sure it's properly aligned for the new thread, but future work should likely make rfork_thread a little easier to use by ensuring proper alignment.
Some trivial cleanup has been done with a couple of error writes, moving strings into char arrays for use with the less fragile sizeof().