MEDIUM: checks: Add check-alpn.

Add a way to configure the ALPN used by check, with a new "check-alpn"
keyword. By default, the checks will use the server ALPN, but it may not
be convenient, for instance because the server may use HTTP/2, while checks
are unable to do HTTP/2 yet.
diff --git a/include/types/checks.h b/include/types/checks.h
index 364eee8..6346fe3 100644
--- a/include/types/checks.h
+++ b/include/types/checks.h
@@ -186,6 +186,8 @@
 	struct sockaddr_storage addr;   	/* the address to check */
 	struct wait_event wait_list;            /* Waiting for I/O events */
 	char *sni;				/* Server name */
+	char *alpn_str;                         /* ALPN to use for checks */
+	int alpn_len;                           /* ALPN string length */
 };
 
 struct check_status {