Provide a new WPACKET function for filling in all the lengths

For the psk extension we need to fill in all the lengths of the message so
far, even though we haven't closed the WPACKET yet. This provides a function
to do that.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2259)