Fix BSD and macOS builds w.r.t. pthread_setname_np (bug#38632)

Fix BSD and macOS builds w.r.t. pthread_setname_np (bug#38632)

pthread_setname_np takes only a single argument on BSD and macOS,
and affects the current thread only.

* configure.ac: Add check for single-argument pthread_setname_np
* src/systhread.c (sys_thread_set_name): New (w32 and pthread versions).
(sys_thread_create): Remove name argument and name-setting.
(w32_beginthread_wrapper): Remove name-setting.
* src/systhread.h (sys_thread_create, sys_thread_set_name):
Update prototypes.
* src/thread.c (run_thread): Call sys_thread_set_name.
(Fmake_thread): Adapt call to sys_thread_create.
* src/thread.h (struct thread_state): Adjust comment.