trace: rename the trace channel types

Since the term 'channel' is already used as synonym for a BIO object attached
to a trace category, having a 't_channel' channel type and a 't_callback' channel
type somehow overburdens this term. For that reason the type enum constants are
renamed to 'SIMPE_CHANNEL' and 'CALLBACK_CHANNEL'.
(The conversion to capital letters was done to comply to the coding style.)

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8463)