Complete among multiple auth-source-search choices when creating. Don't show the...

Complete among multiple auth-source-search choices when creating.  Don't show the password when prompting for creation.

* auth-source.el (auth-source-search): Updated docs to talk about
multiple creation choices.
(auth-source-netrc-create): Accept a list as a value (from the search
parameters) and do completion on that list.  Keep a separate netrc line
with the password obscured for showing the user.