Instead of every command resolving the target (-t or -s) itself, prepare

the state (client, session, winlink, pane) for it it before entering the
command. Each command provides some flags that tell the prepare step
what it is expecting.

This is a requirement for having hooks on commands (for example, if you
hook "select-window -t1:2", the hook command should to operate on window
1:2 not whatever it thinks is the current window), and should allow some
other target improvements.

The old cmd_find_* functions remain for the moment but that layer will
be dropped later.

Joint work with Thomas Adam.

cmd-show-environment.c

@@ -37,7 +37,7 @@ const struct cmd_entry cmd_show_environment_entry = {
 	"show-environment", "showenv",
 	"gst:", 0, 1,
 	"[-gs] " CMD_TARGET_SESSION_USAGE " [name]",
-	0,
+	CMD_SESSION_T,
 	cmd_show_environment_exec
 };
 
@@ -86,18 +86,13 @@ enum cmd_retval
 cmd_show_environment_exec(struct cmd *self, struct cmd_q *cmdq)
 {
 	struct args		*args = self->args;
-	struct session		*s;
 	struct environ		*env;
 	struct environ_entry	*envent;
 
 	if (args_has(self->args, 'g'))
 		env = global_environ;
-	else {
-		s = cmd_find_session(cmdq, args_get(args, 't'), 0);
-		if (s == NULL)
-			return (CMD_RETURN_ERROR);
-		env = s->environ;
-	}
+	else
+		env = cmdq->state.tflag.s->environ;
 
 	if (args->argc != 0) {
 		envent = environ_find(env, args->argv[0]);