busctl: split out introspection parser from tree logic so that we can reuse it for a future "busctl introspect" command
set: make set_consume() actually free the allocated string if the string already is in the set
tests: fix minor memory leak
core: watchdog bus properties cannot be both writable and constant
sd-bus: refuse properties that claim to be both writable and constant at the same time
busctl: add new "introspect" verb for introspecting objects
busctl: improve output of service creds
busctl: pass error output to stdout (rather than stderr) when generate tree for all objects

This is a ton more useful when some services fail, since we continue
crawling then and output everything to a pager.