posix: internal: make priority / policy transforms available

Two functions can be used with relative ease to convert between
Zephyr and POSIX priorities and policies. Namely,

uint32_t zephyr_to_posix_priority(int32_t z_prio, int *policy)
int32_t posix_to_zephyr_priority(uint32_t priority, int policy)

These are not necessarily public API, but they helped with the
POSIX Philosophers Sample, which is in a subsequent commit.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>