efi_loader: correct signature of GetPosition, SetPosition

efi_loader: correct signature of GetPosition, SetPosition

The UEFI spec requires that file positions are passed as u64 in
GetPosition() and SetPosition().

Check if the file handle points to a directory in GetPosition().

Provide a unit test for GetPosition() and SetPosition().

Fix Coverity warning CID 184079 (CONSTANT_EXPRESSION_RESULT).

Add comments.

Fixes: b6dd57773719 ("efi_loader: use correct types in EFI_FILE_PROTOCOL")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>