bug#25061: consider adding %COMPAT to default gnutls priority string

[Eli Zaretskii <eliz@gnu.org>]

> From: Ted Zlatanov <tzz@lifelogs.com>
> Cc: wingo@igalia.com,  25061@debbugs.gnu.org,  ludo@gnu.org,  michael.albinus@gmx.de,  larsi@gnus.org
> Date: Sat, 09 Dec 2017 18:50:08 -0500
> 
> I've looked at the code and at the work that Michael has kindly done on
> connection-local variables and profiles.
> 
> Eli, first, I need to know if I can make large changes (introducing
> connection profiles to GnuTLS) this close to the release. If so, I'll
> work in emacs-26.
> 
> If we can't, I'll do this work for 26.2 and work in master.

Is everything you proposed strictly needed to fix the original
problem?  It sounds like a lot of changes just for having the
equivalent of '(setq gnutls-algorithm-priority "NORMAL:%DUMBFW")'
setting in effect.  E.g., could we simply make the above the default
value of gnutls-algorithm-priority for Emacs 26.1, and do all the rest
on master?

Previously, you said:

> I'd rather stay with the current defaults and defer the rest of the work
> to when connection-specific settings are available.

but you didn't explain why you didn't like changing the defaults.  If
that's because you thought it wasn't very clean, then would such a
change be okay just as a temporary measure, to be replaced by a
cleaner solution in Emacs 27?  Or can this default hurt something?

Thanks.