Rework CMS_add_simple_smimecap()

Rework CMS_add_simple_smimecap()

This is an API to add an OID attribute to the set of SMIMECapabilities.
While attributes are complicated in general, this only supports simple
capabilities encoded as an OID with an optional integer parameter (e.g.,
the key size of a cipher).

Make this API transactional, i.e., don't leave a new empty set behind on
failure or leak the key size if setting the parameter on the X509_ALGOR
fails.

Also convert to single exit and add a doc comment with a reference.

ok beck