Customize displaying of ERC buffers on reconnect

Customize displaying of ERC buffers on reconnect

* lisp/erc/erc-backend.el (erc--server-last-reconnect-count):
Add variable to record last reconnect tally.

* lisp/erc/erc.el (erc-reconnect-display): Add new option to specify
channel-buffer display behavior on reconnect.
(erc-setup-buffer): Use option `erc-reconnect-display' if warranted.
(erc-cmd-JOIN): Forget last reconnect count when issuing a manual
/JOIN command.
(erc-connection-established): Record reconnect count in internal var
before resetting.
(Bug#51753)