Add memfd_create(2) and mlock2(2).

These are old enough now that the latest devices will have kernels that
support them.

Also add basic doc comments to <sys/mman.h>.

Test: treehugger
Change-Id: I1b5ff5db0b6270f5c374287cac1d6a751a0259f5
diff --git a/tests/dlext_test.cpp b/tests/dlext_test.cpp
index 67ebf37..59cf2f7 100644
--- a/tests/dlext_test.cpp
+++ b/tests/dlext_test.cpp
@@ -29,9 +29,7 @@
 #include <android-base/file.h>
 #include <android-base/strings.h>
 
-#include <linux/memfd.h>
 #include <sys/mman.h>
-#include <sys/syscall.h>
 #include <sys/types.h>
 #include <sys/vfs.h>
 #include <sys/wait.h>
@@ -942,7 +940,7 @@
   const std::string lib_path = GetTestlibRoot() + "/libtest_simple.so";
 
   // create memfd
-  int memfd = syscall(__NR_memfd_create, "foobar", MFD_CLOEXEC);
+  int memfd = memfd_create("foobar", MFD_CLOEXEC);
   if (memfd == -1 && errno == ENOSYS) {
     return;
   }