Windows - libethash - multiple concurrent miners crash application

[ghost]

Attempting to run multiple separate miners in Windows (7/8/10) crashes out. After initial miner started any subsequent miner will fail when attempting to open the DAG. This isn't happening in Linux.

The issue is that the ethash_fopen function in io_win32.c calls fopen_s which opens files exclusively in Windows environment. I suggest we could change:

FILE* ethash_fopen(char const* file_name, char const* mode)
{
	FILE* f;
	return fopen_s(&f, file_name, mode) == 0 ? f : NULL;
}

to

FILE* ethash_fopen(char const* file_name, char const* mode)
{
	return _fsopen( file_name, mode, _SH_DENYNO );
}

I've changed and tested this locally and have had no issues so far with it.

Let me know if you'd like my to open a pull request with the change in place for review. I'm new to contributing to github projects so I'm not up to speed on good etiquette...