Revive option erc-query-on-unjoined-chan-privmsg

Revive option erc-query-on-unjoined-chan-privmsg

* etc/ERC-NEWS: Mention reinstated and renamed legacy option
`erc-query-on-unjoined-chan-privmsg' as well as a change in behavior
for `erc-auto-query', when nil.  Also fix erroneous ChangeLog
reference in 5.5 section.
* lisp/erc/erc-backend.el (erc-server-PRIVMSG): Consider flag
`erc-receive-query-display-defer' and revived option
`erc-query-unjoined-chan-privmsg' when deciding whether to create a
new query buffer.  And only "open" a buffer for an unknown target when
the latter option is non-nil.
* lisp/erc/erc.el (erc-cmd-QUERY): Make error more informative.
(erc-query): Revise deprecation message.
(erc-auto-query, erc-receive-query-display): Swap alias and aliased
and add option to `erc-buffers' group.  Mention the nonstandard
meaning of nil and update package-version to signify a behavioral
change, even though the default value remains untouched.
(erc-receive-query-display-defer): Add new variable, a compatibility
switch to access legacy behavior for `erc-auto-query'.
(erc-query-on-unjoined-chan-privmsg,
erc-ensure-target-buffer-on-privmsg): Revise doc string and add alias.
Change package-version to ERC 5.6 due to slightly refined meaning.
* test/lisp/erc/erc-scenarios-base-attach.el: New file.
* test/lisp/erc/resources/base/channel-buffer-revival/reattach.eld:
New file.  (Bug#62833)