Prefer erc-target to erc-default-target

Prefer erc-target to erc-default-target

* etc/ERC-NEWS: Mention `erc-target' and new `erc-server-buffer-p'
alias.
* lisp/erc/erc-backend.el (erc-process-sentinel): Set `joined-p' slot
of `erc--target-channel' object to nil when applicable.
(erc-server-JOIN): Mark `erc--target-channel' object as being joined.
* lisp/erc/erc-common.el (erc--target-channel): Add `joined-p' slot.
Use hyphenated name so accessor function's name ends in "joined-p"
rather than "joinedp".  Note that this will always be nil when
disconnected.
(erc--target): Relocate here from erc.el.
(erc-target): New public API function to return the current buffer's
target as a string, even in channels that have been unjoined.
* lisp/erc/erc-networks.el (erc--default-target): Remove forward
declaration.
(erc-networks--id-reload): Use `erc-target' instead of
`erc--default-target' as predicate for visiting target buffers.
* lisp/erc/erc.el (erc-remove-channel-users): Set channel "joinedness"
to nil in `erc--target-channel' object, when applicable.
(erc--target): Move to erc-common.
(erc--default-target): Remove, replaced by new function `erc-target'.
(erc-query-buffer-p): Use `erc-target'.
(erc-after-connect): Revise doc string.
(erc-connection-established): Revise doc string and move
`erc-unhide-query-prompt' business before hook.
(erc--current-buffer-joined-p): Remove comment and use new `joined-p'
slot of `erc--target-channel' for determining "joinedness" of channel.
(erc-kill-buffer-function): Use `erc--target-channel-p' for detecting
whether the buffer is a channel buffer.
* test/lisp/erc/erc-networks-tests.el
(erc-networks--shrink-ids-and-buffer-names--hook-collapse-target):
Remove comment.
* test/lisp/erc/erc-scenarios-base-reuse-buffers.el
(erc-scenarios-common--base-reuse-buffers-channel-buffers):
Clarify assertion.
* test/lisp/erc/erc-tests.el (erc-with-all-buffers-of-server):
Replace `erc-default-recipients' with `erc--target'.
(erc--target-from-string): Update expected shape of
`erc--target-channel' struct with new `joined-p' slot.
(erc-message): Set `erc--target' in buffer "#chan".  (Bug#66578)