Added support for internal_access for C#

Sync the manually integrate changes in google3/third_party

Tidy up for C# codegen

ObjC String followups: one test case only addition, behavior change for invalid UTF-8

Update the simulators used for some tests under Xcode 7.3 to be iOS 9.3.

This also renames generate_directories to base_namespace_specified; generating directories is the
immediate *effect* of specifying a base namespace, but with this change the options reflect what has been
specified rather than the effect. (There may be other effects in the future, of course.)

This should have no behavioral changes at all.

This doesn't strictly enforce an 80-column limit, but removes the most egregious violations.
The indentation in the C# generator code is inconsistent in general, unfortunately - if we have
any good tools that can be trusted to reformat, I'd be happy to apply them.

This seems to be some code evolution side effects. Back when there was a custom
string class, we couldn't really error when we finally saw the string was bad
so we had to return the empty string, but now that full validation is done
up front, it can error out.

Added CLI option internal_access for types (C#)

Do not link in pthread library for Android builds.

* `csharp_options`: Added `Options` to encapsulate generator options.
   Supported options for now - file_extension, base_namespace
* `{Blah}Generator`: Now accept `Options*` as parameter to constructor
* `csharp_generator.cc`: Parse and populate options
* `Makefile.am`: Added `csharp_options.h`
* `extract_includes.bat.in`: Added `csharp_options.h`

Refactoring code to two commits. This is the first commit

This is required to allow Tensorflow to build on Android without hacks. Currently we create a dummy pthread library just to satisfy this dependency for a library that does not exist on Android. See https://github.com/google/protobuf/issues/1373 for more context.

Added deprecated option handling to objective-c

Linking the cpp implementation extension statically with libprotobuf

Remove duplicate test cases.

Add -position and -isAtEnd for use when manually parsing input streams.

(NCrunch noticed these.)

Introduce a new nuget package, Google.Protobuf.Tools, basically to contain protoc on multiple platforms.

Added an API to allow oversize protos when using C++ extension in Python

Replace #include <iostream> with #include <ostream> in int128.cc

Introduce a new nuget package, Google.Protobuf.Tools, basically to contain protoc on multiple platforms.

I've moved both protoc.exe and the proto files out of Google.Protobuf.
The .proto files aren't a slam-dunk, but it feels like they belong with protoc as you'd *use* them with protoc.
It's not clear to me whether we really need both an x86 and x64 version of protoc.exe, as x86 would work on 64-bit Windows anyway. Discuss :)

Refactoring of FieldDescriptor

iostream is not actually necessary here, and it introduces unnecessary
static initializers.

Use the T() instead of NULL for the default value.

Add support for running on Jenkins

This makes no externally visible behavioral changes. Internally and non-behaviorally:

- We use a field (compiler-generated) to store the JsonName to avoid recomputing it repeatedly
- The documentation for JsonName is updated to reflect the meaning better
- Readonly autoprops and expression-bodied properties used where possible

csharp: add support for the json_name option

The template can be specialized on primitives, e.g. double, where
converting NULL will trigger a warning.