[1.12.x] Backport various fixes to dbus-1.12
dbus-1.12 backports of selected fixes:
- !255 (merged) (#374 (closed), #378 (closed))
- !256 (merged)
- "bus/signal: fix compilation when only -Dembedded-tests" from !218 (merged)
Requires !259 (merged), if we want CI to pass, which we do.