Using errno to detect ethash_io failures

- Printf debug output is still printed by ethash itself unless one
  compiles with -DETHASH_NO_CRITICAL_OUTPUT.

internal.c

@@ -364,6 +364,7 @@ static bool ethash_mmap(struct ethash_full* ret, FILE* f)
 {
 	int fd;
 	char* mmapped_data;
+	errno = 0;
 	ret->file = f;
 	if ((fd = ethash_fileno(ret->file)) == -1) {
 		return false;

io.c

@@ -21,6 +21,7 @@
 #include "io.h"
 #include <string.h>
 #include <stdio.h>
+#include <errno.h>
 
 enum ethash_io_rc ethash_io_prepare(
 	char const* dirname,
@@ -32,6 +33,8 @@ enum ethash_io_rc ethash_io_prepare(
 {
 	char mutable_name[DAG_MUTABLE_NAME_MAX_SIZE];
 	enum ethash_io_rc ret = ETHASH_IO_FAIL;
+	// reset errno before io calls
+	errno = 0;
 
 	// assert directory exists
 	if (!ethash_mkdir(dirname)) {

io.h

@@ -59,6 +59,7 @@ enum ethash_io_rc {
  * figure out what kind of problem (I/O, memory e.t.c.) causes a NULL
  * ethash_full_t
  */
+#ifndef ETHASH_NO_CRITICAL_OUTPUT
 #define ETHASH_CRITICAL(...)							\
 	do													\
 	{													\
@@ -66,6 +67,9 @@ enum ethash_io_rc {
 		printf("\n");									\
 		fflush(stdout);									\
 	} while (0)
+#else
+#define ETHASH_CRITICAL(...)          
+#endif
 
 /**
  * Prepares io for ethash