Fix freopen() where the path is null.

This has been in the standard since C99, but we've never supported it
before. It's apparently used by SPIRV-Tools.

I tried implementing this the other way (with fcntl(2)) first, but
eventually realized that that's more complicated and gives worse
results. This implementation assumes that /proc is mounted, but so much
of libc relies on that at this point that I don't think there's any
realistic case where the fcntl(2) implementation would be preferable,
and there are many where it's not.

The fact that no-one's mentioned this until now suggests that it's not a
heavily used feature anyway.

I've also replaced AssertCloseOnExec() with a CloseOnExec()
boolean-valued function instead, because it's really annoying getting
assertion failures that don't point you at the test line in question,
and instead point to some common helper code.

Test: treehugger
Change-Id: Ia2e53bf2664a4f782581042054ecd492830e2aed
diff --git a/tests/unistd_test.cpp b/tests/unistd_test.cpp
index 41ca8b4..595ba90 100644
--- a/tests/unistd_test.cpp
+++ b/tests/unistd_test.cpp
@@ -1166,10 +1166,10 @@
 TEST(UNISTD_TEST, dup3) {
   int fd = open("/proc/version", O_RDONLY);
   ASSERT_EQ(666, dup3(fd, 666, 0));
-  AssertCloseOnExec(666, false);
+  ASSERT_FALSE(CloseOnExec(666));
   close(666);
   ASSERT_EQ(667, dup3(fd, 667, O_CLOEXEC));
-  AssertCloseOnExec(667, true);
+  ASSERT_TRUE(CloseOnExec(667));
   close(667);
   close(fd);
 }