[1.12] Fix error behaviour on reload if a connection has an unknown uid
Backport of !417 (merged) and !439 (merged), which need to be reviewed first. Includes !438 (merged), which needs to be reviewed first.
I'm in two minds about whether this is too intrusive for 1.12.x, which at this point is basically security-fixes-only - but I did make an exception for !438 (merged), because crashing the system bus is a really bad failure mode.
Changes relative to !439 (merged):
- Omit
tests/CMakeLists.txt
change since that file didn't exist in 1.12.x. I didn't try to add the new test to the old version of the CMake build, because 1.12.x primarily exists for the benefit of older LTS Linux distributions, which all use Autotools. - Adjust the changes in test code to apply to where those tests lived in 1.12.x.