Revert "socket-proxyd: Add --listener option for listener/destination pairs."

This reverts commit adcf4c81c58511b67644e17fa743d1729d3c9ccf.

We have a better solution for the problem of making two processes run in
the same namespace, and --listener is not needed hence and should be
dropped.

Conflicts:
	man/systemd-socket-proxyd.xml
man: get rid of shell script example in systemd-socket-proxyd man page
man: move socket-proxyd man page to section 8, since it is not in the $PATH
util: fix handling of trailing whitespace in split_quoted()

Inspired by a patch by Lukas Nykryn.
bus: add new sd_bus_creds object to encapsulate process credentials

This way we can unify handling of credentials that are attached to
messages, or can be queried for bus name owners or connection peers.

This also adds the ability to extend incomplete credential information
with data from /proc,

Also, provide a convenience call that will automatically determine the
most appropriate credential object for an incoming message, by using the
the attached information if possible, the sending name information if
available and otherwise the peer's credentials.
clean up TODO