Make GnuPG version check robuster

Make GnuPG version check robuster

We changed the default gpg program to "gpg2" from "gpg" in the commit
f93d669a16bd3cb3f43f0c8cfd22fe18b627a6a1.  However, there are two
maintained branches (2.0 and 2.1) and Emacs doesn't work well with 2.0
series.  Check the actual version of "gpg2" at run time, and properly
divert to "gpg" if necessary.
* lisp/epg-config.el: Require 'cl-lib for `cl-destructuring-bind'.
(epg-config--program-alist): New variable.
(epg--configurations): New variable.
(epg-configuration-find): New function.
(epg-config--make-gpg-configuration): New function.
(epg-config--make-gpgsm-configuration): New function.
(epg-configuration): Mark as obsolete.
* lisp/epg.el (epg-context): Use `epg-configuration-find'.