lib/posix: correct the meaning of CONFIG_MAX_PTHREAD_COUNT

Current code implement CONFIG_MAX_PTHREAD_COUNT as the maximum number
of POSIX threads that can ever be created, rather than the maximum
number of active POSIX threads. Use pthread_state of struct posix_thread
to track the state of posix thread in posix_thread_pool so that we can
reuse the unused posix thread.

Fixes #15516.

Signed-off-by: Wentong Wu <wentong.wu@intel.com>
3 files changed