Remove testing::internal::posix:: functions in C++11 standard

Assuming that the C++11 compiler requirement translates into a C++11-compliant standard library as well, I think we can remove some of the functions in `gtest-port.cc`.

* FOpen - std::fopen in ``
* FClose - std::fclose in ``
* FReopen - std::freopen in ``
* StrError - std::strerror in ``
* Abort - std::abort in ``

@gennadiycivil Would you be interested in a PR / CL that removes these functions?

Start in gtest-port.cc and inspect the definitions and references for FOpen, FClose, FReopen, StrError, and Abort. Confirm the corresponding standard-library headers and calls, then verify that the wrappers and their uses are removed without breaking the existing build and tests.

Written by the indexing model from the issue text.