kcgi_writer_putc —
    write into a writer
Returns an enum kcgi_err indicating the
    error state.
  - KCGI_OK
- Success (not an error).
- KCGI_ENOMEM
- Internal memory allocation failure.
- KCGI_HUP
- The output connection has been terminated. For FastCGI connections, the
      current connection should be released with
      khttp_free(3)
      and parse loop reentered.
- KCGI_FORM
- The connection is still expecting headers with
      khttp_head(3).
      Indicates that
      khttp_body(3)
      did not return with success or was not invoked. For FastCGI connections,
      the current connection should be released with
      khttp_free(3)
      and parse loop reentered.
- KCGI_SYSTEM
- Internal system error writing to the output stream.