bug-reference auto-setup for IRC, implementation for ERC

bug-reference auto-setup for IRC, implementation for ERC

* lisp/progmodes/bug-reference.el (bug-reference-setup-from-irc-alist):
Change SERVER-REGEXP to NETWORK-REGEXP in docstring.
* lisp/progmodes/bug-reference.el (bug-reference--maybe-setup-from-irc):
Change semantics from requiring a match of channel OR server to
requiring a match of both (if both are configured).
* lisp/progmodes/bug-reference.el (bug-reference-try-setup-from-erc):
New defun doing the auto-setup for ERC.
(bug-reference--run-auto-setup): Run bug-reference-try-setup-from-erc.
* etc/NEWS: Extend entry describing bug-reference auto-setup.