bug#46979: Allow setting Man-default-entry via Local Variables

[Eli Zaretskii]

> From: 積丹尼 Dan Jacobson
>  <jidanni@jidanni.org>
> Date: Sun, 07 Mar 2021 07:18:12 +0800
> 
> Well wouldn't it be great if one could do e.g.,
> 
> # Local Variables:
> # Man-default-entry: "crontab(5)"
> # End:
> 
> in one's .crontab file.
> 
> Or a
> 
> # Local Variables:
> # Man-default-entry: "bash"
> # End:
> 
> that one could put in their .bashrc file.

IMO, it would make much more sense to derive that from the current
major-mode (in Man-default-man-entry).  Patches welcome.