Additional explanations for SSL_ERROR_WANT_READ/WRITE.

=item SSL_ERROR_WANT_READ, SSL_ERROR_WANT_WRITE

The operation did not complete; the same TLS/SSL I/O function should be

called again later.  There will be protocol progress if, by then, the

underlying B<BIO> has data available for reading (if the result code is

B<SSL_ERROR_WANT_READ>) or allows writing data (B<SSL_ERROR_WANT_WRITE>). 

For socket B<BIO>s (e.g. when SSL_set_fd() was used) this means that

select() or poll() on the underlying socket can be used to find out

when the TLS/SSL I/O function should be retried.

called again later.  If, by then, the underlying B<BIO> has data

available for reading (if the result code is B<SSL_ERROR_WANT_READ>)

or allows writing data (B<SSL_ERROR_WANT_WRITE>), then some TLS/SSL

protocol progress will take place, i.e. at least part of an TLS/SSL

record will be read or written.  Note that the retry may again lead to

a B<SSL_ERROR_WANT_READ> or B<SSL_ERROR_WANT_WRITE> condition.

There is no fixed upper limit for the number of iterations that

may be necessary until progress becomes visible at application

protocol level.

For socket B<BIO>s (e.g. when SSL_set_fd() was used), select() or

poll() on the underlying socket can be used to find out when the

TLS/SSL I/O function should be retried.

Caveat: Any TLS/SSL I/O function can lead to either of

B<SSL_ERROR_WANT_READ> and B<SSL_ERROR_WANT_WRITE>, i.e. SSL_read()

may want to write data and SSL_write() may want to read data.

B<SSL_ERROR_WANT_READ> and B<SSL_ERROR_WANT_WRITE>.  In particular,

SSL_read() may want to write data and SSL_write() may want to read

data.  This is because TLS/SSL handshakes may occur at any time

during the protocol (initiated by either the client or the server);

SSL_read() and SSL_write() will handle any pending handshakes.

=item SSL_ERROR_WANT_X509_LOOKUP