fix fclose of permanent (stdin/out/err) streams

this fixes a bug reported by Nuno Gonçalves. previously, calling
fclose on stdin or stdout resulted in deadlock at exit time, since
__stdio_exit attempts to lock these streams to flush/seek them, and
has no easy way of knowing that they were closed.

conceptually, leaving a FILE stream locked on fclose is valid since,
in the abstract machine, it ceases to exist. but to satisfy the
implementation-internal assumption in __stdio_exit that it can access
these streams unconditionally, we need to unlock them.

it's also necessary that fclose leaves permanent streams in a state
where __stdio_exit will not attempt any further operations on them.
fortunately, the call to fflush already yields this property.

src/stdio/fclose.c

@@ -9,7 +9,7 @@ int fclose(FILE *f)
 	int r;
 	int perm;
 	
-	FFINALLOCK(f);
+	FLOCK(f);
 
 	__unlist_locked_file(f);
 
@@ -26,6 +26,7 @@ int fclose(FILE *f)
 
 	if (f->getln_buf) free(f->getln_buf);
 	if (!perm) free(f);
-	
+	else FUNLOCK(f);
+
 	return r;
 }