core: For OpenPGP let offline mode disable dirmngr.
* src/engine-gpg.c (struct engine_gpg): New flag.offline. (gpg_set_engine_flags): Set it. Also fix setting of no_symkey_cache. (build_argv): Pass --disable-dirmngr in offline mode.
- GnuPG-bug-id: T3831
- Signed-off-by: Werner Koch <wk@gnupg.org>