Add a command queue to standardize and simplify commands that call other

commands and allow a command to block execution of subsequent
commands. This allows run-shell and if-shell to be synchronous which has
been much requested.

Each client has a default command queue and commands are consumed one at
a time from it. A command may suspend execution from the queue by
returning CMD_RETURN_WAIT and then resume it by calling cmd_continue() -
for example run-shell does this from the callback that is fired after
the job is freed.

When the command queue becomes empty, command clients are automatically
exited (unless attaching). A callback is also fired - this is used for
nested commands in, for example, if-shell which can block execution of
the client's cmdq until a new cmdq becomes empty.

Also merge all the old error/info/print functions together and lose the
old curclient/cmdclient distinction - a cmdq is bound to one client (or
none if in the configuration file), this is a command client if
c->session is NULL otherwise an attached client.

server-fn.c

@@ -194,7 +194,7 @@ server_status_window(struct window *w)
 
 	/*
 	 * This is slightly different. We want to redraw the status line of any
-	 * clients containing this window rather than any where it is the
+	 * clients containing this window rather than anywhere it is the
 	 * current window.
 	 */
 
@@ -563,7 +563,7 @@ int
 server_set_stdin_callback(struct client *c, void (*cb)(struct client *, int,
     void *), void *cb_data, char **cause)
 {
-	if (c == NULL) {
+	if (c == NULL || c->session != NULL) {
 		*cause = xstrdup("no client with stdin");
 		return (-1);
 	}