xmpp.el - an Emacs interface to rexmpp

Since Emacs can't poll sockets and use rexmpp directly, this works by
introducing a program which basically serializes library calls and
callbacks into XML, and which xmpp.el runs as a subprocess.

Once rexmpp_xml_interface is built, and possibly the path to it is
adjusted in the xmpp-command variable, one can load xmpp.el, set a
password in ~/.authinfo (or elsewhere auth-source will be able to read
it from, with "port xmpp"), perhaps enable tracking-mode, and run M-x
xmpp RET <JID> RET.

The buffers it creates are an XML console, a text console (using
rexmpp's console module, type "help" to see the available commands), a
process buffer, a log buffer. The xmpp-query function (or just
incoming messages) will create query buffers for one-to-one chats,
xmpp-muc-join creates MUC ones.

The used XML interface will probably be adjusted, and there's still a
lot to add or improve in xmpp.el, but it is fairly usable (i.e.,
replaces bitlbee + rcirc for me) since September 2021.

Possibly in the future it will work with other libraries as well,
and/or will be moved out of the rexmpp's repository.