Add helper for restoring local session vars in ERC

Add helper for restoring local session vars in ERC

* lisp/erc/erc-common.el (erc--input-split): Suppress warning for
obsolete variable `erc-send-this' in init form.
* lisp/erc/erc-goodies.el (erc--keep-place-indicator-setup): Use macro
`erc--restore-initialize-priors' to preserve last session's indicator
position, if any.
(erc-keep-place-indicator-mode, erc-keep-place-indicator-enable): Use
convenience function to show missing-dependency notice.
* lisp/erc/erc-sasl.el (erc-sasl-auth-source-password-as-host): Merge
redundant `when' forms for clarity.
(erc-sasl--init): Remove unused function.
(erc-sasl-mode, erc-sasl-enable): Use helper to restore
`erc-sasl--options', essentially inlining the body of the now defunct
`erc-sasl--init'.
* lisp/erc/erc.el (erc--restore-initialize-priors): New macro to help
local modules and mode hooks prefer existing state over initializing
anew.
(erc--warn-once-before-connect): Add helper function to display
an "error notice" just after module setup.
(erc-accidental-paste-threshold-seconds) Improve doc string.
* test/lisp/erc/erc-goodies-tests.el
(erc-controls-highlight--examples, erc-controls-highlight--inverse,
erc-controls-highlight--motd, erc-keep-place-indicator-mode): Remove
feature check.  For the latter, also start fake process and shadow
`erc-connect-pre-hook'.
* test/lisp/erc/erc-tests.el (erc--restore-initialize-priors): New
test.  Also see test/lisp/erc/erc-scenarios-base-local-modules.el for
a more realistic exercising of this functionality.  (Bug#60936)