MINOR: checks: Store the proxy in checks.

Instead of assuming we have a server, store the proxy directly in struct
check, and use it instead of s->server.
This should be a no-op for now, but will be useful later when we change
mail checks to avoid having a server.

This should be backported to 1.9.
diff --git a/include/types/checks.h b/include/types/checks.h
index 6346fe3..f89abcb 100644
--- a/include/types/checks.h
+++ b/include/types/checks.h
@@ -180,6 +180,7 @@
 	int send_string_len;			/* length of agent command string */
 	char *send_string;			/* optionally send a string when connecting to the agent */
 	struct server *server;			/* back-pointer to server */
+	struct proxy *proxy;                    /* proxy to be used */
 	char **argv;				/* the arguments to use if running a process-based check */
 	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 */