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.

tmux.c

@@ -330,8 +330,6 @@ main(int argc, char **argv)
 	options_init(&global_w_options, NULL);
 	options_table_populate_tree(window_options_table, &global_w_options);
 
-	ARRAY_INIT(&cfg_causes);
-
 	/* Enable UTF-8 if the first client is on UTF-8 terminal. */
 	if (flags & IDENTIFY_UTF8) {
 		options_set_number(&global_s_options, "status-utf8", 1);