Do not open login shell (-s & -ss option)

[greymd]

Unify all the work logs about following features.

  -s: Speedy mode: Run commands promptly. But panes are not interactive.
  -ss: Speedy mode and close windows automatically when process ends.

-s option is came from "speedy" or "sh" or something.
I believe this feature solves both #89 #90 .

[greymd]

Memo: tmux behavior of split-window command.
https://github.com/tmux/tmux/blob/7448b38327f856dae6ce915eae63f7f6c3f036e0/window.c#L983

   * If given one argument, assume it should be passed to sh -c;
   * with more than one argument, use execvp(). If there is no
   * arguments, create a login shell.

https://github.com/tmux/tmux/blob/master/cmd-split-window.c#L122

	shell = options_get_string(s->options, "default-shell");
	if (*shell == '\0' || areshell(shell))
		shell = _PATH_BSHELL;

https://github.com/tmux/tmux/blob/master/compat.h#L65

#define	_PATH_BSHELL	"/bin/sh"

If default-shell is defined, it is used with execl.
-c is also hardcoded.

https://github.com/tmux/tmux/blob/master/cmd-split-window.c#L122

			execl(wp->shell, argv0, "-c", first, (char *)NULL);

Otherwise /bin/sh is used with execvp.

That's why following command does not open login shell.

$ tmux split-window "ls; sleep 1"

Memo: Document about exec series is here.
https://gist.github.com/fffaraz/8a250f896a2297db06c4

[greymd]

If you want "to execute commands as is, promptly, on the existing pane", then -sex option is required.
Like this.

$ xpanes -sex 'echo '{01..15}

[greymd]

I am now working on this feature on develop-3.1.0 branch and touch upon the new options.
Finally, I am pretty sure that suppressing closing pane should be default to improve usability.

Let me change the specification of this feature like following.
Abolish -a option. set -ss option instead.

Before:

-s: option:  Speedy mode. Just execute command. Do not make panes interactive.
-a: keep alive option (do not close pane when the command finished when -s option)-s: option:  Speedy mode. Just execute command. D

After:

  -s: Speedy mode: Run commands promptly. But panes are not interactive.
  -ss: Speedy mode and close windows automatically when process ends.

There are three reasons:

1. xpanes --help displays all the options alphabetical order.
   If -a option is going to be placed upper part of the --help/man page,
   it is difficult for users to notice the relationship between new option and -s option.

2. It is common to use same letters sequentially in Unix commands.
   i.e: ssh -vvv option.

3. easier to type than -a

[greymd]

This feature is released at v3.1.0

[greymd]