INAPPROPRIATE USE OF sched_lock() BY pthreads

```
Description: In implementation of standard pthread functions, the non-
standard, NuttX function sched_lock() is used. This is very
strong since it disables pre-emption for all threads in all
task groups. I believe it is only really necessary in most
cases to lock threads in the task group with a new non-
standard interface, say pthread_lock().

This is because the OS resources used by a thread such as
mutexes, condition variable, barriers, etc. are only
meaningful from within the task group. So, in order to
performance exclusive operations on these resources, it is
only necessary to block other threads executing within the
task group.

This is an easy change: pthread_lock() and pthread_unlock()
would simply operate on a semaphore retained in the task
group structure. I am, however, hesitant to make this change:
In the FLAT build model, there is nothing that prevents people
from accessing the inter-thread controls from threads in
different task groups. Making this change, while correct,
might introduce subtle bugs in code by people who are not
using NuttX correctly.
Status: Open
Priority: Low. This change would improve real-time performance of the
OS but is not otherwise required.
```