win32/win32.c (socklist_insert, socklist_lookup, socklist_delete):
new functions to wrap of st_insert(), st_lookup() and st_delete() to
allocating socklist is deferred until it is really needed.
win32/win32.c (exit_handler): delete socklist only if it is
win32/win32.c (rb_w32_sysinit, StartSockets): refactoring: move
initialization of select_mutex to StartSockets().
win32/win32.c (exit_handler): refactoring: delete select_mutex only
if winsock is used.