Add inside-Emacs mode to GUI pinentry programs
* configure.ac: Add --enable-pinentry-emacs and --enable-inside-emacs option. (BUILD_LIBPINENTRY_EMACS): New conditional. (BUILD_PINENTRY_EMACS): New conditional. (INSIDE_EMACS): New conditional. * Makefile.am (pinentry_emacs): New. (SUBDIRS): Add "emacs" subdir if PINENTRY_EMACS is set. * pinentry/pinentry-emacs.h: New file. * pinentry/pinentry-emacs.c: New file. * pinentry/Makefile.am: New file. * pinentry/pinentry.c (option_handler): Handle the allow-emacs-prompt Assuan option. * emacs/pinentry-emacs.c: New file. * emacs/Makefile.am: New file.