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.

cmd-kill-server.c

@@ -27,7 +27,7 @@
  * Kill the server and do nothing else.
  */
 
-enum cmd_retval	 cmd_kill_server_exec(struct cmd *, struct cmd_ctx *);
+enum cmd_retval	 cmd_kill_server_exec(struct cmd *, struct cmd_q *);
 
 const struct cmd_entry cmd_kill_server_entry = {
 	"kill-server", NULL,
@@ -40,7 +40,7 @@ const struct cmd_entry cmd_kill_server_entry = {
 };
 
 enum cmd_retval
-cmd_kill_server_exec(unused struct cmd *self, unused struct cmd_ctx *ctx)
+cmd_kill_server_exec(unused struct cmd *self, unused struct cmd_q *cmdq)
 {
 	kill(getpid(), SIGTERM);