Removed GPR_POSIX_FILE, and changed the files that used it

BUILD

CMakeLists.txt

Makefile

binding.gyp

build.yaml

config.m4

gRPC-Core.podspec

grpc.gemspec

include/grpc/impl/codegen/port_platform.h

package.xml

...security/credentials/google_default/credentials_generic.c

...security/credentials/google_default/credentials_windows.c

...y/credentials/google_default/google_default_credentials.h

src/python/grpcio/grpc_core_dependencies.py

test/core/security/credentials_test.c

tools/doxygen/Doxyfile.core.internal

tools/run_tests/sources_and_headers.json

vsprojects/vcxproj/grpc/grpc.vcxproj

vsprojects/vcxproj/grpc/grpc.vcxproj.filters