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.1

@@ -3468,7 +3468,11 @@ Miscellaneous commands are as follows:
 .Bl -tag -width Ds
 .It Ic clock-mode Op Fl t Ar target-pane
 Display a large clock.
-.It Ic if-shell Ar shell-command command Op Ar command
+.It Xo Ic if-shell
+.Op Fl b
+.Ar shell-command command
+.Op Ar command
+.Xc
 .D1 (alias: Ic if )
 Execute the first
 .Ar command
@@ -3477,19 +3481,27 @@ if
 returns success or the second
 .Ar command
 otherwise.
+With
+.Fl b ,
+.Ar shell-command
+is run in the background.
 .It Ic lock-server
 .D1 (alias: Ic lock )
 Lock each client individually by running the command specified by the
 .Ic lock-command
 option.
 .It Xo Ic run-shell
+.Fl b
 .Op Fl t Ar target-pane
 .Ar shell-command
 .Xc
 .D1 (alias: Ic run )
 Execute
 .Ar shell-command
-in the background without creating a window.
+without creating a window.
+With
+.Fl b ,
+the command is run in the background.
 After it finishes, any output to stdout is displayed in copy mode (in the pane
 specified by
 .Fl t