cipher: Add option to specify salt length for PSS verification.

* cipher/pubkey-util.c (_gcry_pk_util_data_to_mpi): Check for
salt-length token.

Add possibility to use a different salt length for RSASSA-PSS
verification instead of the default 20.

Additional changes by wk:

• Detect overlong salt-length
• Release LIST on error.

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