gpg: Assert that an opaque parameter is really what we expect.
* g10/gpg.h (SERVER_CONTROL_MAGIC): New const. (server_control_s): Add field 'magic'. * g10/gpg.c (gpg_init_default_ctrl): Init MAGIC. * g10/import.c (impex_filter_getval): Assert MAGIC.
- Signed-off-by: Werner Koch <wk@gnupg.org>