Preserve point when inserting date stamps in ERC

Preserve point when inserting date stamps in ERC

* lisp/erc/erc-stamp.el (erc-stamp-mode, erc-stamp-disable): Move
remaining local teardown business to `erc-stamp--setup' and use
`erc-buffer-do' instead of `erc-with-all-buffers-of-server' to
emphasize that all ERC buffers are affected.
(erc-stamp--insert-date-stamp-as-phony-message): Move `erc--msg-props'
binding to `erc-stamp--lr-date-on-pre-modify'.
(erc-stamp--lr-date-on-pre-modify): Bind `erc--msg-props' here so that
the related guard condition in `erc-add-timestamp' is satisfied and
`erc-insert-timestamp-function' runs.  This fixes a regression new in
ERC 5.6 and introduced by c68dc778 "Manage some text props for ERC
insertion-hook members".  Also, `save-excursion' when narrowing to
prevent point from being dislodged after submitting input at the
prompt.
(erc-insert-timestamp-left-and-right): Don't initialize date stamps
when `erc-timestamp-format-left' is nil or consists only of newlines,
and enable fallback behavior in such cases on behalf of users without
informing them.  Allow global hook members to run first so that those
owned by `scrolltobottom' and similar can see the unadulterated input.
Fix wrong hook name.
(erc-stamp--setup): Fix wrong hook name.  Kill all local vars here
instead of sharing this duty with the minor-mode toggle.  (Bug#60936)