During the initial handshake or subsequent renegotiation, the protocol
might need to read in order to write (or conversely to write in order
to read). It might be blocked from doing so by the state of the
underlying socket (i.e. there is no data to read, or there is no space
to write).
The library indicates this condition by returning one of the errors
`MBEDTLS_ERR_SSL_WANT_READ` or `MBEDTLS_ERR_SSL_WANT_WRITE`. When that
happens, we need to enforce that the next poll operation only considers
the direction that the library indicated.
In addition, mbedtls does its own read buffering that we need to take
into account while polling, and we need to save the last error between
read()/write() and ioctl().
