Run erc-services-regain-mode callback on a timer

Run erc-services-regain-mode callback on a timer

* lisp/erc/erc-services.el (erc-services-regain-timeout-seconds): New
variable.
(erc-services-regain-mode): Mention CertFP in doc string.
(erc--nickname-in-use-make-request): Ensure the 900 RPL_LOGGEDIN
callback always runs after `erc-services-regain-timeout-seconds', even
when SASL isn't being used.
* test/lisp/erc/erc-scenarios-services-misc.el
(erc-scenarios-services-misc--regain-command/oftc): New test.
* test/lisp/erc/resources/services/regain/taken-regain-oftc.eld: New
file.

(cherry picked from commit a62a26239780684d481533a57d12de520b737f9b)