Mostly align BIO_read()/BIO_write() return values with OpenSSL 3.x.

Mostly align BIO_read()/BIO_write() return values with OpenSSL 3.x.

For various historical reasons, there are a number of cases where our
BIO_read() and BIO_write() return slightly different values to what
OpenSSL 3.x does (of course OpenSSL 1.0 differs from OpenSSL 1.1 which
differs from OpenSSL 3.x). Mostly align these - some further work will be
needed.

Issue raised by tb@ who also wrote some test code.