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

@@ -336,8 +336,6 @@ 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);