Explicitly validate the permuted register count when encoding and decoding.

This adds a new PLCRASH_ASYNC_CFE_PERMUTATION_REGISTER_MAX constant that
may be used distinctly from the PLCRASH_ASYNC_CFE_SAVED_REGISTER_MAX
constant -- while CFE supports up to PLCRASH_ASYNC_CFE_SAVED_REGISTER_MAX
values, the permuted register encoding supports fewer.