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.

Makefile

@@ -60,12 +60,10 @@ SRCS=	arguments.c \
 	cmd-select-pane.c \
 	cmd-select-window.c \
 	cmd-send-keys.c \
-	cmd-send-prefix.c \
 	cmd-server-info.c \
 	cmd-set-buffer.c \
 	cmd-set-environment.c \
 	cmd-set-option.c \
-	cmd-show-buffer.c \
 	cmd-show-environment.c \
 	cmd-show-messages.c \
 	cmd-show-options.c \
@@ -80,6 +78,7 @@ SRCS=	arguments.c \
 	cmd-unbind-key.c \
 	cmd-unlink-window.c \
 	cmd.c \
+	cmd-queue.c \
 	colour.c \
 	control.c \
 	control-notify.c \