Disable buffering when writing to mysqld's stdin.

Due to a bug in Visual Studio 2015 runtime, some newlines get lost
which makes the bootstrapping fail (which also makes MSI installer
non-functional).

This does not have a visible effect on packages we produce so far,
because we do not use VS2015 yet for building them.

sql/mysql_install_db.cc

@@ -563,6 +563,10 @@ static int create_db_instance()
   if (!in)
     goto end;
 
+  if (setvbuf(in, NULL, _IONBF, 0))
+  {
+    verbose("WARNING: Cannot disable buffering on mysqld's stdin");
+  }
   if (fwrite("use mysql;\n",11,1, in) != 1)
   {
     verbose("ERROR: Cannot write to mysqld's stdin");