bug#28603: 25.3; Certificate authority on macOS is empty

[Mark Ferlatte]

When running (package-refresh-contents) on macOS, MELPA returns a TLS
error. This ended up being due to gnutls-trustfiles not having a working
default entry for macOS. Adding /etc/ssl/cert.pem to gnutls-trustfiles
resolves the issue.

I believe that adding that to the default settings in lisp/net/gnutls.el
would be a helpful change.



In GNU Emacs 25.3.1 (x86_64-apple-darwin13.4.0, NS appkit-1265.21 Version 
10.9.5 (Build 13F1911))
of 2017-09-12 built on builder10-9.local
Windowing system distributor 'Apple', version 10.3.1561
Configured using:
'configure --with-ns '--enable-locallisppath=/Library/Application
Support/Emacs/${version}/site-lisp:/Library/Application
Support/Emacs/site-lisp' --with-modules'

Configured features:
NOTIFY ACL GNUTLS LIBXML2 ZLIB TOOLKIT_SCROLL_BARS NS MODULES

Important settings:
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Major mode: EL

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Recent messages:
Loading /Users/ferlatte/.emacs.d/savefile/recentf...done
Loading Prelude’s modules...
Loading /Users/ferlatte/.emacs.d/prelude-modules.el (source)...done
Loading personal configuration files in /Users/ferlatte/.emacs.d/personal...
Loading /Users/ferlatte/.emacs.d/personal/custom.el (source)...done
Loading /Users/ferlatte/.emacs.d/personal/init.el (source)...done
Prelude is ready to do thy bidding, Master ferlatte!
Starting new Ispell process aspell with default dictionary...
For information about GNU Emacs and the GNU system, type C-h C-a.
Prelude tip: Press <C-c G> to search in GitHub.

Load-path shadows:
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/Applications/Emacs.app/Contents/Resources/lisp/emacs-lisp/seq
/Users/ferlatte/.emacs.d/elpa/let-alist-1.0.5/let-alist hides 
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Memory information:
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