*) SSL_shutdown() never returns -1, on error it returns 0.
   This fixes incidental "bad write retry" errors.
*) cleaning stale global SSL error
*) remove SSL_MODE_ACCEPT_MOVING_WRITE_BUFFER, we never need it

fix STARTTLS prompt in SMTP

fix merge_slashes

large_client_header_buffers did not free()ed before keep-alive

improve throughput with large limit_rate

use usec and pid as start value

the simple expression has the same precision without overflow

omit unnecessary test, because NGX_ERROR == -1,
but r->headers_out.last_modified_time != -1 at this point

If-Range support