gpgsm: Allow parsing of PKCS#12 files with two private keys.

* sm/minip12.c (struct p12_parse_ctx_s): Add privatekey2.
(parse_shrouded_key_bag): Handle a second private key.
(p12_parse_free_kparms): New.
* sm/import.c (parse_p12): Factor some code out to ...
(p12_to_skey): this.
(parse_p12): Use p12_parse_free_kparms.

Take care: We allow parsing of a second private key but we are not yet
able to import the second private key.

The whole things is required to at least import the certificates of
current pkcs#12 files as created by the German Elster tax system. No
test data, sorry.