* pthread_stop_world.c (pthread_sigmask): Undefine before using
in GC_print_sig_mask() (only if DEBUG_THREADS); add the comment.
* win32_threads.c (dlopen, _beginthread): Don't undefine (since
neither redirected nor used here).
* win32_threads.c (GC_Thread_Rep): Rename "table_management" to
"tm" for short; remove "tm_" prefix.
* win32_threads.c (in_use, next): Don't define the macros; use
tm.in_use and tm.next fields, respectively (to ease debugging).
* win32_threads.c (HASH): Rename to PTHREAD_MAP_HASH (to have
unique name across the project).