Make erc-compat.el obsolete

Make erc-compat.el obsolete

* lisp/erc/erc-compat.el: Move from here...
* lisp/obsolete/erc-compat.el: ...to here.
* lisp/erc/erc-backend.el:
* lisp/erc/erc-pcomplete.el:
* lisp/erc/erc-stamp.el:
* lisp/erc/erc-track.el:
* lisp/erc/erc.el: Don't require 'erc-compat'.

* lisp/erc/erc-backend.el (erc-decode-string-from-target):
* lisp/erc/erc-dcc.el (pcomplete/erc-mode/DCC):
* lisp/erc/erc-fill.el (erc-fill-mode):
* lisp/erc/erc-goodies.el (erc-controls-interpret):
* lisp/erc/erc-log.el (erc-log-setup-logging):
* lisp/erc/erc-notify.el (erc-notify-QUIT):
* lisp/erc/erc.el (erc-startup-file-list, define-erc-module)
(erc-canonicalize-server-name, erc-cmd-SV, erc-banlist-update)
(erc-group-list, erc-seconds-to-string): Adjust callers.
* lisp/erc/erc.el: Require cl-lib and format-spec.

* etc/NEWS: Add entry announcing erc-compat.el being marked as
obsolete.