Two locations had the same loop for writing out a value. Pull it out into
a function.
Reviewed-by: NRich Salz <rsalz@openssl.org>

This is an internal API. Some of the tests were for programmer erorr and
"should not happen" situations, so a soft assert is reasonable.
Reviewed-by: NRich Salz <rsalz@openssl.org>

A few style tweaks here and there. The main change is that curr and
packet_len are now offsets into the buffer to account for the fact that
the pointers can change if the buffer grows. Also dropped support for the
WPACKET_set_packet_len() function. I thought that was going to be needed
but so far it hasn't been. It doesn't really work any more due to the
offsets change.
Reviewed-by: NRich Salz <rsalz@openssl.org>

Reviewed-by: NRich Salz <rsalz@openssl.org>

The PACKET documentation is already in packet_locl.h so it makes sense to
have the WPACKET documentation there as well.
Reviewed-by: NRich Salz <rsalz@openssl.org>

Also added the WPACKET_cleanup() function to cleanup a WPACKET if we hit
an error.
Reviewed-by: NRich Salz <rsalz@openssl.org>

Reviewed-by: NRich Salz <rsalz@openssl.org>

No need to declare an explicit sub-packet. Just start one.
Reviewed-by: NRich Salz <rsalz@openssl.org>

To avoid confusion with the read PACKET structure.
Reviewed-by: NRich Salz <rsalz@openssl.org>

Reviewed-by: NRich Salz <rsalz@openssl.org>