Use _FILE_OFFSET_BITS macro to distinguish between native MinGW and
32 bit under MinGW-64. The latter does not require fseeko/ftello
redefinition which it already does in case of _FILE_OFFSET_BITS=64.
We need to hide "modprobe efivars" error output to avoid confusion. So
consolidate grub_util_exec_* into single function that can optionally redirect
all three standard descriptors and make all other functions compatibility
wrappers.
Also remove include/grub/osdep/exec_unix.h which does not appear to be used
anywhere.
the function of these files exceeds what can be sanely handled in shell
in posix-comaptible way. Also writing it in C extends the functionality
to non-UNIX-like OS and minimal environments.