Don't change thread priority if it is default under Unix

Skip all the code dealing with the priority/scheduling parameters if the
priority is just wxPRIORITY_DEFAULT anyhow, as it's unnecessary to do
anything in this case and it can result in spurious errors.

Also extract this code into a separate SetThreadPriority() function to
make wxThreadInternal::Create() itself shorter and more clear.

Closes #18195.

docs/changes.txt

@@ -96,6 +96,7 @@ All:
 - Make wxList and wxVector iterators conform to input iterator requirements.
 - Fix MT-safety problem when reading and writing from wxSocket (jkubalik).
 - Fix build issues under HaikuOS (mill-j).
+- Avoid spurious errors on thread creation under NetBSD.
 
 All (GUI):