Add mini modules bufbar and nickbar to ERC

Add mini modules bufbar and nickbar to ERC

* doc/misc/erc.texi: Add `bufbar' and `nickbar' to known modules.
Also add `keep-place', which was missing, and remove `bbdb', which is
not part of ERC.  Add new section "Auxiliary Modules" for listing
experimental modules or those typically managed by some other feature.
* erc/ERC-NEWS: Mention new mini modules for libraries
erc-status-sidebar.el and erc-speedbar.el.
* lisp/erc/erc.el (erc-modules): Add `bufbar' and `nickbar' to
selection of offered modules.
* test/lisp/erc/erc-tests.el (erc-tests--modules): Add `bufbar' and
`nickbar'.  (Bug#63595)