ssl/npn+alpn: adjust protocol selection to fail if no protocols match

The current behavior causes a server written using rust-openssl to (if
it cannot negotiate a protocol) fallback to the first protocol it has
avaliable.

This makes it impossible to detect protocol mismatches.

This updates our selection to be more similar to how openssl's
s_server behaves: non-matching protocols are not supplied with a
fallback.

Note that some setups may actually want a fallback protocol supplied
via ALPN. To support those cases, we should consider adding a generic
callback that allows protocol selection to be entirely controlled by
the programmer.

For the purposes of having a sane default, however, not supplying a
default (and mimicing s_server's behavior) is the best choice.