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
needed.

• Signed-off-by: Werner Koch <wk@gnupg.org>