reconsider k_mem_pool APIs

[zephyrbot]

Reported by Andrew Boie:

Currently, k_mem_pool works with k_mem_block objects. These k_mem_block objects have a pointer to the memory blob plus some kernel bookkeeping information in the embedded struct k_mem_block_id. We do not want malfeasance with k_mem_block_id to be able to mess up the kernel.

Ideally, the entire mem_pool APIs would be usable from userspace without any privilege elevation, and only k_malloc() and k_free() however, irq_lock()/unlock() calls in the code confound this.

Determine how to best adapt memory pool APIs in a system with user/supervisor threads.

(Imported from Jira ZEP-2333)