mailutils: extend send_email with BCC option

• Adds new parameter bccaddr to send_email() that allows clients to specify addresses to be put in BCC.
  
  This acts as a workaround for the silent behaviour of mailutils that multiple recipients in the To field are put in BCC instead.
  
  This means that you can now specify one receiver in To: and the rest in BCC for cases where one would want to do this.

• Also updates bst_send_mail with the new API behaviour.

• Does some code de-duplication efforts and pep8 updates.