src/openssl.c: Add attributes methods on x509.csr

[telent]

What does it do?

Add methods on CSR to deal with attributes

getAttribute(name, startpos=-1) =>
  (first attribute with that name after startpos) and pos as multiple values
    
getAttributeTypes() => list of all the attribute names
    
addAttribute(name, values, type = MBSTRING_ASC)
  values is an array of strings
  type (if supplied) is numeric, one of MBSTRING_ASC, MBSTRING_UTF8 etc

  only stringish types are implemented

deleteAttribute(pos)
  deletes the attribute at position pos (as returned by getAttribute) 

This is submitted for feedback not for merge yet. Obvious warts

• only supports attributes with string types (although this is probably 95% of practical uses for attributes, maybe this is not a big deal)
• to specify a type other than MBSTRING_ASC you need to look up its value in openssl header files

I would welcome any and all comment/criticism about api design, coding standards, etc etc

Why is is useful?

My use case for this is that I want a program that replicates the Puppet CA "policy based keysigning" functionality. The client generates a CSR with a pre-shared key embedded, and submits it for signing using an API: the API server checks for that PSK and decides whether or not to sign it. The convention seems to be to use the challengePassword custom attribute for this purpose, so I needed a way of getting to it.