The idea is that the application shall be able to register output
channels or callbacks to print tracing output as it sees fit.

OpenSSL internals, on the other hand, want to print thoses texts using
normal printing routines, such as BIO_printf() or BIO_dump() through
well defined BIOs.

When the application registers callbacks, the tracing functionality
sets up an internal BIO that simply forwards received text to the
appropriate application provided callback.
Co-authored-by: NDr. Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
Reviewed-by: NPaul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/8198)