epg: Support key editing

* epg.el (epg-context): New slot EDIT-CALLBACK.
(epg--process-filter): Call EDIT-CALLBACK when editing a key.
(epg-reset): Reset EDIT-CALLBACK of the context.
(epg-start-edit-key): New function.
(epg-edit-key): New function.
Use automatic variables in nt/Makefile.in

* nt/Makefile.in (addpm${EXEEXT}, ddeclient${EXEEXT})
(cmdproxy${EXEEXT}, runemacs${EXEEXT}, emacs.res):
Use automatic variables.