OPENSSL_NO_XXX cleanup: OPENSSL_NO_BUF_FREELISTS

Remove OPENSSL_NO_BUF_FREELISTS. This was turned on by default,
so the work here is removing the 'maintain our own freelist' code.
Also removed a minor old Windows-multibyte/widechar conversion flag.

Reviewed-by: Andy Polyakov <appro@openssl.org>