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-string.c

@@ -59,7 +59,8 @@ cmd_string_ungetc(size_t *p)
  * string, or NULL for empty command.
  */
 int
-cmd_string_parse(const char *s, struct cmd_list **cmdlist, char **cause)
+cmd_string_parse(const char *s, struct cmd_list **cmdlist, const char *file,
+    u_int line, char **cause)
 {
 	size_t		p;
 	int		ch, i, argc, rval;
@@ -131,7 +132,7 @@ cmd_string_parse(const char *s, struct cmd_list **cmdlist, char **cause)
 			if (argc == 0)
 				goto out;
 
-			*cmdlist = cmd_list_parse(argc, argv, cause);
+			*cmdlist = cmd_list_parse(argc, argv, file, line, cause);
 			if (*cmdlist == NULL)
 				goto out;