epg: Use epg-context-set-*-callback for extra treatment of args

* epa.el (epa-sign-file, epa-encrypt-file, epa-decrypt-region)
(epa-sign-region, epa-encrypt-region): Use
`epg-context-set-{passphrase,progress}-callback', instead of
`setf'.  This partially reverts commit 9e48a95c (bug#19150).
Reported by José A. Romero L.