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/libc/libc.map.txt b/libc/libc.map.txt
index c3b9e2c..9b39bb8 100644
--- a/libc/libc.map.txt
+++ b/libc/libc.map.txt
@@ -1493,6 +1493,8 @@
     cnd_signal;
     cnd_timedwait;
     cnd_wait;
+    memfd_create;
+    mlock2;
     mtx_destroy;
     mtx_init;
     mtx_lock;