Add PROXY over TLS support to PROXY Stream

This commit makes it possible to use PROXY Stream not only over TCP,
but also over TLS. That is, now PROXY Stream can work in two modes as
far as TLS is involved:

1. PROXY over (plain) TCP - PROXYv2 headers are sent unencrypted before
TLS handshake messages. That is the main mode as described in the
PROXY protocol specification (as it is clearly stated there), and most
of the software expects PROXYv2 support to be implemented that
way (e.g. HAProxy);

2. PROXY over (encrypted) TLS - PROXYv2 headers are sent after the TLS
handshake has happened. For example, this mode is being used (only ?)
by "dnsdist". As far as I can see, that is, in fact, a deviation from
the spec, but I can certainly see how PROXYv2 could end up being
implemented this way elsewhere.

lib/isc/netmgr/netmgr.c

@@ -2353,6 +2353,8 @@ isc_nm_has_encryption(const isc_nmhandle_t *handle) {
 #endif /* HAVE_LIBNGHTTP2 */
 	case isc_nm_streamdnssocket:
 		return (isc__nm_streamdns_has_encryption(handle));
+	case isc_nm_proxystreamsocket:
+		return (isc__nm_proxystream_has_encryption(handle));
 	default:
 		return (false);
 	};
@@ -2456,6 +2458,9 @@ isc_nmsocket_set_tlsctx(isc_nmsocket_t *listener, isc_tlsctx_t *tlsctx) {
 	case isc_nm_streamdnslistener:
 		isc__nm_streamdns_set_tlsctx(listener, tlsctx);
 		break;
+	case isc_nm_proxystreamlistener:
+		isc__nm_proxystream_set_tlsctx(listener, tlsctx);
+		break;
 	default:
 		UNREACHABLE();
 		break;
@@ -2697,6 +2702,10 @@ isc__nmhandle_get_selected_alpn(isc_nmhandle_t *handle,
 	case isc_nm_tlssocket:
 		isc__nmhandle_tls_get_selected_alpn(handle, alpn, alpnlen);
 		return;
+	case isc_nm_proxystreamsocket:
+		isc__nmhandle_proxystream_get_selected_alpn(handle, alpn,
+							    alpnlen);
+		return;
 	default:
 		break;
 	};