Move epa-pinentry-mode to epg-config (Bug#26298)

Move epa-pinentry-mode to epg-config (Bug#26298)

* lisp/epa.el (epa-pinentry-mode): Make into obsolete alias for...
* lisp/epg-config.el (epg-pinentry-mode): ...this new user option.
* lisp/epg.el (epg-context): Use it as the default value for
pinentry-mode.
* etc/NEWS: Announce name change.

* lisp/epa-file.el (epa-file-insert-file-contents)
(epa-file-write-region):
* lisp/epa.el (epa-decrypt-file, epa-sign-file)
(epa-encrypt-file, epa-decrypt-region)
(epa-sign-region, epa-encrypt-region): Don't change
epg-context-pinentry-mode from the default.