core: Implement recpstring option parsing for gpgsm.
* src/engine-gpg.c (append_args_from_recipients_string): Detect bad options. * src/engine-gpgsm.c (set_recipients_from_string): Implement option parsing.
The only option we actually implement is "--" but the code layout is
now very simlar to engine-gpg and can easily be extended if ever
- Signed-off-by: Werner Koch <email@example.com>