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

[Andy Wingo <wingo@igalia.com>]

There have been reports of errors from people using melpa and so on
which manifest themselves as:

    gnutls.c: [0] (Emacs) fatal error: The TLS connection was non-properly terminated.

However I think maybe that's just the symptom and not the cause; see the
previous report:

    https://debbugs.gnu.org/cgi/bugreport.cgi?bug=25060

Ludovic Courtès was seeing a similar issue to the one that people are
reporting for melpa etc in Guix, where we also use GnuTLS though not in
Emacs.  He then found that GNU wget, which also uses GnuTLS, wasn't
exhibiting the same behavior.  He was eventually able to reproduce the
problem with just gnutls-cli.  He tracked down the difference in that if
he adds %COMPAT to the priority list, then he has no problems:

    https://lists.gnu.org/archive/html/bug-guix/2016-04/msg00098.html
    http://bugs.gnu.org/23311

Note that the problem only exhibits itself for some web sites, and only
some of the time.  It manifested itself as a timeout where the server
would get stuck, which could explain that people are unable to fetch
packages then blame the problem on the spurious post-close error message
from bug 25060.

So, as Ludovic suggests in his message, a workaround might be:

    (setq gnutls-algorithm-priority "NORMAL:%COMPAT")

See Ludovic's message for some justification.  Just an idea.  I have
been trying to reproduce the problem that people report locally as some
TLS errors but I have not been able to.

Andy