smime_keys: Handle certificate chains in add_cert. (closes #3339) (closes #3559)

smime_keys: Handle certificate chains in add_cert. (closes #3339) (closes #3559)

Find all chains in the certificate provided.  For each chain create a
separate leaf and intermediate certificate file.  Because Mutt controls
the label prompt, use a single label for all chains.

Also, loosen up cert file parsing to allow attributes even if they
aren't delimited by a "Bag Attributes" header.

Thanks to David J. Weller-Fahy for his testing and feedback!