REGTEST: Add connection/proxy_protocol_send_unique_id_alpn

REGTEST: Add connection/proxy_protocol_send_unique_id_alpn

This reg-test checks that sending unique IDs via PPv2 works for servers
with the `alpn` option specified (issue #640). As a side effect it also
checks that PPv2 works with ALPN (issue #651).

It has been verified that the test fails without the following commits
applied and succeeds with them applied.

   1f9a4ecea BUG/MEDIUM: backend: set the connection owner to the session when using alpn.
   083fd42d5 BUG/MEDIUM: connection: Ignore PP2 unique ID for stream-less connections
   eb9ba3cb2 BUG/MINOR: connection: Always get the stream when available to send PP2 line

Without the first two commits HAProxy crashes during execution of the
test. Without the last commit the test will fail, because no unique ID
is received.