posix: mutex: abstract pthread_mutex_t as uint32_t

Consistent with the change of `pthread_t` from
`struct posix_thread` to `uint32_t`, we can now also abstract
`pthread_mutex_t` as `uint32_t` and separate `struct posix_mutex`
as an implementation detail, hidden from POSIX API consumers.

This change deprecates `PTHREAD_MUTEX_DEFINE()` in favour of the
(standardized) `PTHREAD_MUTEX_INITIALIZER`.

This change introduces `CONFIG_MAX_PTHREAD_MUTEX_COUNT`.

Signed-off-by: Chris Friedt <cfriedt@meta.com>
6 files changed