bug#19812: 24.4; suggest `shell-mode' not interactive

[Juri Linkov]

> Perhaps somebody else will chime in to say whether they've ever said
> `M-x shell-mode' and it wasn't a mistake.  :-)

It might be useful to visit log files previously saved from shell buffers,
like compilation-mode adds such local-variable prop-line:

  -*- mode: compilation -*-

so compilation log saved to files could be visited later.

But one problem with using

  -*- mode: shell -*-

in files saved from previous shells is that it doesn't highlight prompts -
thus it lacks a very important feature of shell-mode.