Add POSIX <utmpx.h>.

Now <utmpx.h> isn't any more useful on Android than <utmp.h> is, but it
is POSIX, and -- importantly -- we can implement it with just a header
file, so code can use it on every existing API level.

macOS does indeed only have the <utmpx.h> functions (although it does
still have the <utmp.h> header!), so potentially portable code might
want <utmpx.h> on Android. (glibc/musl both have both headers.)

Bug: https://github.com/landley/toybox/pull/213
Test: treehugger
Change-Id: Iaa88167708182009a63e2e1a15f11186b251ed02
diff --git a/tests/utmpx_test.cpp b/tests/utmpx_test.cpp
new file mode 100644
index 0000000..55427a6
--- /dev/null
+++ b/tests/utmpx_test.cpp
@@ -0,0 +1,42 @@
+/*
+ * Copyright (C) 2023 The Android Open Source Project
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *  * Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ *  * Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
+ * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+ * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+ * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#include <gtest/gtest.h>
+
+#include <utmpx.h>
+
+TEST(utmpx, smoke) {
+  // Our utmpx "implementation" just calls the utmp no-op functions.
+  setutxent();
+  utmpx empty = {.ut_type = EMPTY};
+  ASSERT_EQ(NULL, getutxent());
+  ASSERT_EQ(NULL, getutxid(&empty));
+  ASSERT_EQ(NULL, getutxline(&empty));
+  endutxent();
+  ASSERT_EQ(NULL, pututxline(&empty));
+}