Fixes to improve auth-source prompting and don't default user name to anything.

Fixes to improve auth-source prompting and don't default user name to anything.

* auth-source.el (auth-source-creation-prompts): New variable to manage
creation-time prompts.
(auth-source-search): Document it.
(auth-source-format-prompt): Add utility function.
(auth-source-netrc-create): Don't default the user name to
user-login-name.  Use `auth-source-creation-prompts' and some default
prompts for user, host, port, and password (the default generic prompt
remains ugly).