Redo erc-nickname-in-use-functions as a local module

* etc/ERC-NEWS: Mention new module `services-regain'.
* lisp/erc/erc-backend.el: Rename option.
* lisp/erc/erc-services.el (erc-services-regain-alist): Strategies for
regaining a lost nickname on reconnect.  This option, in addition to
the rest of these changes, is a redo of
`erc-nickname-in-use-functions' from commit 8c0c9826 "Add hook to
regain nickname in ERC", which originally stemmed from bug#62044.
(erc-services-retry-nick-on-connect, erc-services-issue-regain,
erc-services-issue-ghost-and-retry-nick): New function variants for
`erc-services-regain-alist.
(erc-services-regain-mode, erc-services-regain-enable,
erc-services-regain-disable): New local module to activate
nick-regaining behavior.
(erc--nickname-in-use-make-request): New method, a services-specific
implementation.
* lisp/erc/erc.el (erc--nickname-in-use-make-request): New generic
function to request alternate nick when first choice is rejected.
(erc-nickname-in-use): Call `erc--nickname-in-use-make-request' to
request alternate nick.
* test/lisp/erc/erc-scenarios-services-misc.el
(erc-scenarios-services-misc--reconnect-retry-nick): Adopt renamed
version of `erc-scenarios-base-renick-auto-regain'.
(erc-scenarios-services-misc--regain-command,
erc-scenarios-services-misc--ghost-and-retry-nick): New tests.
* test/lisp/erc/resources/services/regain/reconnect-retry-again.eld:
New test data file reusing existing blob c0529052 that once lived at
resources/base/renick/regain/normal-again.eld.
* test/lisp/erc/resources/services/regain/reconnect-retry.eld: New
test data file reusing existing blob 9f4df70e5 that once lived at
resources/base/renick/regain/normal.eld.
* test/lisp/erc/resources/services/regain/taken-ghost.eld: New test
data file.
* test/lisp/erc/resources/services/regain/taken-regain.eld New test
data file.

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