lib: posix: internal: use a more generic INIT mask and inlines

Previously `PTHREAD_MUTEX_MASK_INIT` was used to mark a
`pthread_mutex_t` as initialized.

The same needs to be done for `pthread_cond_t` and likely others.

Rather than copy-pasting that and a number of inlines that
duplicate the same functionality, simply make it more generic.

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