fopen_internal: return ENOENT on file not found

This is actually expected by open(2), but I do not expect the BIOS
implementation to conform.

libpsx/src/libc.c

@@ -177,6 +177,7 @@ static FILE *fopen_internal(const char *path, const char *mode, FILE *f)
 
 	if(fd == -1)
 	{
+		errno = ENOENT;
 		if(s!=NULL)free(s);
 		return NULL;
 	}