The first step towards a SSL_peek fix.

The main thing to verify about these changes is that nothing at all
has changed, as far as behaviour is concerned (except that some
SSLerr() invocations now have a different function code): SSL_read
(ssl2_read, ssl3_read) behaves exactly as before, and SSL_peek refuses
to do any work exactly as before.  But now the functions actually
doing the work have a 'peek' flag, so it should be easy to change them
to behave accordingly.