Allow ERC modules to extend erc-nick-popup-alist

Allow ERC modules to extend erc-nick-popup-alist

* etc/ERC-NEWS: Mention changes to `erc-button-alist' and superficial
changes to `erc-nick-popup-alist'.
* lisp/erc/erc-button.el (erc-nick-popup-alist): Change type to prefer
associating strings with functions instead of arbitrary sexps.
(erc-button-cmd-KICK, erc-button-cmd-MSG): New functions to serve as
interrogative wrappers for `erc-cmd-KICK' and `erc-cmd-MSG' in
`erc-nick-popup-alist'.  The first also fixes a bug in which all but
the first token of a given "reason" would be omitted from the
":trailing" portion of an outgoing "KICK" message.
(erc-button--nick-popup-alist): New variable to help built-in modules
expose special actions to `erc-nick-popup' without touching
`erc-nick-popup-alist'.
(erc-nick-popup): Present members from both `erc--nick-popup-alist'
and `erc-nick-popup-alist' to the invoking user.  Accommodate
functions as well as arbitrary sexps.  (bug#63569)