-) using dupenv_s instead of getenv_s and calling strdup ourselves.
-) few impossible-to-obtain if checks.
-) various signed/unsigned casting.
-) using time_t instead of time32_t
-) checking output of FormatMessage for failures.
-) don't redefine _WIN32_WINNT without undefining it first.
-) fixed msvc's interlocked casting.
-) renamed AddPort to AddListeningPort.
-) added protobuf's third_party includes to search path.
-) added a missing definition for inet_ntop in mingw32.
-) removed useless declarations.

For the 2-argument version of CreateChannel.

This is a temporary step until #711 is ready to roll out.

And reflects the C++ API in ServerBuilder.

Fixing a few issues in case you *cough* have a different local implementation of
protobuf, gtest and gflags for some weird reason.