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);
 	}