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.
