Fix ALPN

* Perform ALPN after the SNI callback; the SSL_CTX may change due to
  that processing
* Add flags to indicate that we actually sent ALPN, to properly error
  out if unexpectedly received.
* document ALPN functions
* unit tests

Backport of commit 817cd0d5

Reviewed-by: Emilia Käsper <emilia@openssl.org>
Reviewed-by: Dr. Stephen Henson <steve@openssl.org>