Add C++ support for using external FD

Makefile

diff --git a/Makefile b/Makefile
index 78900ae..58358e2 100644
--- a/Makefile
+++ b/Makefile
@@ -3431,6 +3431,7 @@
     src/cpp/client/client_context.cc \
     src/cpp/client/create_channel.cc \
     src/cpp/client/create_channel_internal.cc \
+    src/cpp/client/create_channel_posix.cc \
     src/cpp/client/credentials.cc \
     src/cpp/client/generic_stub.cc \
     src/cpp/client/insecure_credentials.cc \
@@ -3446,6 +3447,7 @@
     src/cpp/server/server_builder.cc \
     src/cpp/server/server_context.cc \
     src/cpp/server/server_credentials.cc \
+    src/cpp/server/server_posix.cc \
     src/cpp/util/byte_buffer.cc \
     src/cpp/util/slice.cc \
     src/cpp/util/status.cc \
@@ -3459,6 +3461,7 @@
     include/grpc++/client_context.h \
     include/grpc++/completion_queue.h \
     include/grpc++/create_channel.h \
+    include/grpc++/create_channel_posix.h \
     include/grpc++/generic/async_generic_service.h \
     include/grpc++/generic/generic_stub.h \
     include/grpc++/grpc++.h \
@@ -3486,6 +3489,7 @@
     include/grpc++/server.h \
     include/grpc++/server_builder.h \
     include/grpc++/server_context.h \
+    include/grpc++/server_posix.h \
     include/grpc++/support/async_stream.h \
     include/grpc++/support/async_unary_call.h \
     include/grpc++/support/byte_buffer.h \
@@ -3786,6 +3790,7 @@
     src/cpp/client/client_context.cc \
     src/cpp/client/create_channel.cc \
     src/cpp/client/create_channel_internal.cc \
+    src/cpp/client/create_channel_posix.cc \
     src/cpp/client/credentials.cc \
     src/cpp/client/generic_stub.cc \
     src/cpp/client/insecure_credentials.cc \
@@ -3801,6 +3806,7 @@
     src/cpp/server/server_builder.cc \
     src/cpp/server/server_context.cc \
     src/cpp/server/server_credentials.cc \
+    src/cpp/server/server_posix.cc \
     src/cpp/util/byte_buffer.cc \
     src/cpp/util/slice.cc \
     src/cpp/util/status.cc \
@@ -3814,6 +3820,7 @@
     include/grpc++/client_context.h \
     include/grpc++/completion_queue.h \
     include/grpc++/create_channel.h \
+    include/grpc++/create_channel_posix.h \
     include/grpc++/generic/async_generic_service.h \
     include/grpc++/generic/generic_stub.h \
     include/grpc++/grpc++.h \
@@ -3841,6 +3848,7 @@
     include/grpc++/server.h \
     include/grpc++/server_builder.h \
     include/grpc++/server_context.h \
+    include/grpc++/server_posix.h \
     include/grpc++/support/async_stream.h \
     include/grpc++/support/async_unary_call.h \
     include/grpc++/support/byte_buffer.h \