Merge of r5004, r5019-r5025: ssl fixes.

Merge of r5004, r5019-r5025: ssl fixes.

*) SSL: speedup loading of configs with many ssl servers.  The patch
   saves one EC_KEY_generate_key() call per server{} block by informing
   OpenSSL about SSL_OP_SINGLE_ECDH_USE we are going to use before
   the SSL_CTX_set_tmp_ecdh() call.

   For a configuration file with 10k simple server{} blocks with SSL
   enabled this change reduces startup time from 18s to 5s on a slow
   test box here.

*) SSL: removed conditions that always hold true.

*) SSL: resetting of flush flag after the data was written.  There is
   no need to flush next chunk of data if it does not contain a buffer
   with the flush or last_buf flags set.

*) SSL: preservation of flush flag for buffered data.  Previously,
   if SSL buffer was not sent we lost information that the data
   must be flushed.

*) SSL: calculation of buffer size moved closer to its usage.
   No functional changes.

*) SSL: avoid calling SSL_write() with zero data size.  According to
   documentation, calling SSL_write() with num=0 bytes to be sent
   results in undefined behavior.

   We don't currently call ngx_ssl_send_chain() with empty chain and
   buffer.  This check handles the case of a chain with total data size
   that is a multiple of NGX_SSL_BUFSIZE, and with the special buffer
   at the end.

   In practice such cases resulted in premature connection close and
   critical error "SSL_write() failed (SSL:)" in the error log.

*) SSL: take into account data in the buffer while limiting output.
   In some rare cases this can result in a more smooth sending rate.

*) SSL: fixed ngx_ssl_handshake() with level-triggered event methods.
   Missing calls to ngx_handle_write_event() and ngx_handle_read_event()
   resulted in a CPU hog during SSL handshake if an level-triggered event
   method (e.g. select) was used.