Make some module toggles more resilient in ERC

Make some module toggles more resilient in ERC

* lisp/erc/erc-goodies.el (erc-scrolltobottom-mode,
erc-scrolltobottom-enable, erc-move-to-prompt-mode,
erc-move-to-prompt-enable): Guard setup procedure behind
`erc--updating-modules-p'.
* lisp/erc/erc-imenu.el (erc-imenu-mode, erc-imenu-enable,
erc-imenu-disable): Don't run setup when `erc--updating-modules-p' is
non-nil.  Also, don't restrict teardown to buffers of the same
process.
* lisp/erc/erc-match.el (erc-match-mode, erc-match-enable): Run
major-mode hook member immediately outside of `erc-update-modules' in
`erc-open'.
* lisp/erc/erc-spelling.el (erc-spelling-mode, erc-spelling-enable):
Only conditionally run setup immediately.
* lisp/erc/erc-stamp.el (erc-stamp-mode, erc-stamp-enable,
erc-stamp-disable): Run setup hook immediately.  Don't forget to
kill local vars in all ERC buffers during teardown.
* lisp/erc/erc.el (erc--updating-modules-p): New variable that global
modules can use to provide their `erc-mode-hook'-deferred code on
demand while shielding it from running during early ERC buffer
initialization.
(erc-open): Make `erc--updating-modules-p' non-nil while activating
global modules.  (Bug#60936)