Use each once in s3_srvr.c to show how they work.

Also fix a bug introduced in c3fc7eea
and made apparent by this change:
ssl3_get_next_proto wasn't updating next_proto_negotiated_len
Reviewed-by: NMatt Caswell <matt@openssl.org>

Rewrite ssl3_get_client_hello to use the new methods.
Reviewed-by: NMatt Caswell <matt@openssl.org>

The PACKET should hold a 'const unsigned char*' underneath as well
but the legacy code passes the record buffer around as 'unsigned char*'
(to callbacks, too) so that's a bigger refactor.
Reviewed-by: NMatt Caswell <matt@openssl.org>

Enhance the PACKET code readability, and fix a stale comment. Thanks
to Ben Kaduk (bkaduk@akamai.com) for pointing this out.
Reviewed-by: NEmilia Käsper <emilia@openssl.org>

Some of the PACKET functions were returning incorrect data. An unfortunate
choice of test data in the unit test was masking the failure.
Reviewed-by: NTim Hudson <tjh@openssl.org>

Provide more robust (inline) functions to replace n2s, n2l, etc. These
functions do the same thing as the previous macros, but also keep track
of the amount of data remaining and return an error if we try to read more
data than we've got.
Reviewed-by: NTim Hudson <tjh@openssl.org>