kernel: fix CONFIG_THREAD_NAME from user mode.

This mechanism had multiple problems:

- Missing parameter documentation strings.
- Multiple calls to k_thread_name_set() from user
  mode would leak memory, since the copied string was never
  freed
- k_thread_name_get() returns memory to user mode
  with no guarantees on whether user mode can actually
  read it; in the case where the string was in thread
  resource pool memory (which happens when k_thread_name_set()
  is called from user mode) it would never be readable.
- There was no test case coverage for these functions
  from user mode.

To properly fix this, thread objects now have a buffer region
reserved specifically for the thread name. Setting the thread
name copies the string into the buffer. Getting the thread name
with k_thread_name_get() still returns a pointer, but the
system call has been removed. A new API k_thread_name_copy()
is introduced to copy the thread name into a destination buffer,
and a system call has been provided for that instead.

We now have full test case coverge for these APIs in both user
and supervisor mode.

Some of the code has been cleaned up to place system call
handler functions in proximity with their implementations.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>

kernel/include/kernel_structs.h

@@ -241,7 +241,7 @@ static ALWAYS_INLINE void z_new_thread_init(struct k_thread *thread,
 #endif
 
 #ifdef CONFIG_THREAD_NAME
-	thread->name = NULL;
+	thread->name[0] = '\0';
 #endif
 
 #if defined(CONFIG_USERSPACE)