Nuke NaCl SFI, part 1.

mojo/nacl/sfi/README.md

-About
-=====
-
-This is a prototype for plumbing Mojo into the NaCl sandbox.  It is currently
-insecure (see below), and does not support Mojo functions that return pointers
-(for example, `MojoMapBuffer`).
-
-Currently, SFI NaCl support is not being actively developed, in favor of
-non-SFI NaCl support.
-
-Using
-=====
-
-The SFI NaCl and accompanying tests should be built by default for Linux.
-The boolean indicating if they are built is "mojo_use_nacl", defined inside
-the BUILD files.
-
-This should create a "monacl_shell" executable, capable of running standalone
-SFI nexes.
-
-Additionally, it will create a content handler for SFI nexes -- any Nexe
-with the line "#!mojo mojo:nacl_content_handler" prepended to it will be
-redirected to the SFI content handler when run with the "mojo_shell".
-
-
-Notes
-=====
-
-`nacl_bindings_generator/interface.py` contains a programmatic description of
-the stable Mojo interface.  This will need to be updated as the interface
-changes.  Run `nacl_bindings_generator/generate_nacl_bindings.py` to generate
-the bindings that plumb this interface into the NaCl sandbox.
-
-Security TODO
-=============
-
-* Validate and copy option structures.
-* Protect untrusted buffers passed into Mojo:
-  * `NaClVmIoWillStart/HasEnded`.
-  * volatile accesses to untrusted memory (untrusted code could race).
-* Overflow checking in array bounds validation.