Make our fortify FD_ISSET const, and expose the fd_mask type.

Bug: http://b/70171464
Test: ran tests
Change-Id: Ifbee29d9888ce7cf76b99844dd9700128256a486
diff --git a/tests/sys_select_test.cpp b/tests/sys_select_test.cpp
index 4ad77f0..0eab876 100644
--- a/tests/sys_select_test.cpp
+++ b/tests/sys_select_test.cpp
@@ -179,3 +179,8 @@
 
   DelayedWriteCleanup(pid, fd);
 }
+
+TEST(sys_select, FD_ISSET_const) {
+  const fd_set none = {};
+  ASSERT_FALSE(FD_ISSET(atoi("0"), &none));
+}