Implement <spawn.h>.

As described here:

  http://pubs.opengroup.org/onlinepubs/9699919799/functions/posix_spawn.html

And here:

  http://man7.org/linux/man-pages/man3/posix_spawn.3.html

Bug: N/A (but mentioned in my inbox since 2013)
Test: ran tests
Change-Id: I0b27b2919b660779e3bd8a25fb429527c16dc621
diff --git a/tests/Android.bp b/tests/Android.bp
index fb8b886..7094d77 100644
--- a/tests/Android.bp
+++ b/tests/Android.bp
@@ -105,6 +105,7 @@
         "semaphore_test.cpp",
         "setjmp_test.cpp",
         "signal_test.cpp",
+        "spawn_test.cpp",
         "stack_protector_test.cpp",
         "stack_protector_test_helper.cpp",
         "stack_unwinding_test.cpp",