mingw: replace MSVCRT's fstat() with a Win32-based implementation

mingw: replace MSVCRT's fstat() with a Win32-based implementation

fstat() is the only stat-related CRT function for which we don't have a
full replacement yet (and thus the only reason to stick with MSVCRT's
'struct stat' definition).

Fully implement fstat(), in preparation of implementing a POSIX 2013
compatible 'struct stat' with nanosecond-precision file times.

This allows us also to implement some clever code to handle pipes and
character devices in our own way.

Signed-off-by: Karsten Blees <redacted>
Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>