bug#46777: 28.0.50; ERC: NickServ identification: Prompt for password after other sources, overall simplifications

[Olivier Certner]

When `erc-prompt-for-nickserv-password' is true, don't ignore the
other forms of identification.  Instead, process them first, and
prompt for the password last.  Separate concerns (determination of the
nick to use, of the password to use, and actual message sending).

Note that the user can be interactively prompted for a password on
reception of a Nickserv request, as before (on
`erc-prompt-for-nickserv-password').

This is a follow-up to #45340 (see end of discussion there for additional 
context).

Pull request with the single commit to be posted after the bug is open. 
Changes rebased on master.

-- 
Olivier Certner