MINOR: checks: Add a new keyword to specify a SNI when doing SSL checks.

Add a new keyword, "check-sni", to be able to specify the SNI to be used when
doing health checks over SSL.
diff --git a/include/types/checks.h b/include/types/checks.h
index 283ff3d..3559f2d 100644
--- a/include/types/checks.h
+++ b/include/types/checks.h
@@ -184,6 +184,7 @@
 	char **envp;				/* the environment to use if running a process-based check */
 	struct pid_list *curpid;		/* entry in pid_list used for current process-based test, or -1 if not in test */
 	struct sockaddr_storage addr;   	/* the address to check */
+	char *sni;				/* Server name */
 };
 
 struct check_status {