systemd returns NRestarts=0 for units with LoadState=not-found, making
it indistinguishable from a genuinely loaded unit with zero restarts.
GetNumRestarts now checks LoadState when NRestarts is 0 and returns
ErrValueNotSet for units that don't exist, matching GetMemoryUsage
behavior.
Also adds unit tests for filterErr (all stderr error mapping cases)
and HasValidUnitSuffix (all valid unit types + negative cases).
Updates syncthing test expectation from ErrValueNotSet to nil since
loaded-but-inactive units legitimately have NRestarts=0.
