Don't call erc-auto-query twice on PRIVMSG

Don't call erc-auto-query twice on PRIVMSG

* lisp/erc/erc-backend.el (erc-server-JOIN): Use `erc--open-target'
instead of `erc-join'.
(erc-server-PRIVMSG): Don't call `erc-auto-query' at all, and instead
borrow the portion of its logic that detects when a query buffer
should be created instead of a channel buffer.

* lisp/erc/erc.el (erc-cmd-QUERY): Update the mode line explicitly
after calling `erc-query' in case it's needed after `erc-setup-buffer'
runs. Simplify.
(erc-query, erc--open-target): Replace uses of `erc-query'
with `erc--open-target' and make the former obsolete.  Don't call
`erc-update-mode-line' because `erc-open' already does that.
(erc-auto-query): Make this function obsolete.  It was previously only
used in erc-backend.el and only sewed confusion.
(erc-query-on-unjoined-chan-privmsg): Add note questioning its role.
It was previously only used by the now deprecated `erc-auto-query'.

* test/lisp/erc/erc-scenarios-misc.el
(erc-scenarios-base-mask-target-routing): Add test for server masks.
* test/lisp/erc/resources/base/mask-target-routing/foonet.eld: New file.