Fortran filesystem

For abnormal runtime conditions (empty path for mkdir) etc. that are a definite failure, throw / catch C++ exceptions. This considerably simplifies the code. However, for the case like Clang with Gfortran on Apple where the ABI is incompatible, don't rely on exceptions for essential functionality like checking file existence.

• use CheckPIESupported so that PIE is done right
• eliminate common/windows.c for conciseness
• make tests skip only shaky parts (apple clang + gfortran) rather than whole test.