RT3479: Add UTF8 support to BIO_read_filename()

If we use BIO_new_file(), on Windows it'll jump through hoops to work
around their unusual charset/Unicode handling. it'll convert a UTF-8
filename to UCS-16LE and attempt to use _wfopen().

If you use BIO_read_filename(), it doesn't do this. Shouldn't it be
consistent?

It would certainly be nice if SSL_use_certificate_chain_file() worked.

Also made BIO_C_SET_FILENAME work (rsalz)

Signed-off-by: Rich Salz <rsalz@akamai.com>
Reviewed-by: Andy Polyakov <appro@openssl.org>