correct locking in stdio functions that tried to be lock-free

these functions must behave as if they obtain the lock via flockfile
to satisfy POSIX requirements. since another thread can provably hold
the lock when they are called, they must wait to obtain the lock
before they can return, even if the correct return value could be
obtained without locking. in the case of fclose and freopen, failure
to do so could cause correct (albeit obscure) programs to crash or
otherwise misbehave; in the case of feof, ferror, and fwide, failure
to obtain the lock could sometimes return incorrect results. in any
case, having these functions proceed and return while another thread
held the lock was wrong.

src/stdio/fwide.c

@@ -5,6 +5,8 @@
 
 int fwide(FILE *f, int mode)
 {
-	if (!f->mode) f->mode = NORMALIZE(mode);
-	return f->mode;
+	FLOCK(f);
+	if (!f->mode) mode = f->mode = NORMALIZE(mode);
+	FUNLOCK(f);
+	return mode;
 }