FreeRTOS-Plus-TCP port fixes

[bjsowa]

1. Fixed mutex drop by using vSemaphoreDelete instead of z_free.
2. Added static allocation support to mutexes.
3. Fixed _z_task_detach by pointing it to _z_task_cancel.
4. Fixed freeing task resources. The current implementation does not make much sense.
5. Fixed issues with task cancelling and joining.
6. Used gettimeofday for z_time functions. It doesn't really make sense to use tick count for this. If compiled with newlib for a target that does not support this function, user can just implement _gettimeofday if needed.

[github-actions]

PR missing one of the required labels: {'documentation', 'dependencies', 'breaking-change', 'enhancement', 'internal', 'new feature', 'bug'}

[bjsowa]

I'm still testing some things, will mark this as ready for review once I'm done.