In `create_client_simulation_tls_clienthello()` the variable `sni_extension_found` should be set if the ClientHello includes an SNI extension. Instead it was being set if and only if the ClientHello included some extension other than SNI.
This bug wasn't detected before for two reasons:
It is rare to have a ClientHello that includes an SNI extension, but no other extensions.
The code still works correctly if `sni_extension_found` is set even if there is no SNI in the ClientHello.
So, the bug only creates a problem if the browser's ClientHello include an SNI extension and no other extensions (see "BingPreview Jun 2014" in the client_simulation branch).
This PR fixes two issues related to curve X25519.
First, while OpenSSL 1.1.0 supports curve X25519, it is not included in the output of `$OPENSSL ecparam -list_curves`. I tried several versions of OpenSSL (and one version of LibreSSL), and every version output either "Error with command" or "unknown option" in response to `$OPENSSL s_client -curves $curve` if it either did not support the `-curves` option or did not support `$curve`. (When the `-curve` option was supported with `$curve`, a "connect" error was output.)
The second issue is that the "Server Temp Key" line in the output of `s_client` is different for curve X25519. For other elliptic curves, the output is
```
Server Temp Key: ECDH, P-256, 256 bits
```
For X25519 it is:
```
Server Temp Key: X25519, 253 bits
```
So, `read_dhbits_from_file()` needs to allow for `$what_dh` being "X25519" rather than "ECDH" and `run_pfs()` needs to allow for the possibility that the curve name will be the first field rather than the second.
