Merge github.com:google/grpc into async-api-new

Makefile

build.json

include/grpc++/credentials.h

include/grpc/grpc_security.h

include/grpc/support/port_platform.h

src/core/security/credentials.c

src/core/security/security_context.c

src/core/support/env.h

src/core/support/env_linux.c

src/core/support/env_posix.c

src/core/support/env_win32.c

src/core/support/file.c

src/core/support/file.h

src/core/support/file_posix.c

src/core/support/file_win32.c

src/csharp/README.md

test/core/end2end/fixtures/chttp2_simple_ssl_fullstack.c

test/core/support/env_test.c

test/core/support/file_test.c

tools/run_tests/tests.json