filterErr checked 'does not exist' before 'Interactive authentication
required', so when systemd printed both (common for mask/unmask on
non-installed units as a non-root user), the wrong error was returned.
Reorder checks so permission, bus, and masked errors take priority over
existence warnings. Add tests covering mixed-stderr scenarios.
Also:
- CI: install and start nginx so user + root tests pass
- CI: run tests as both user and root for full coverage
- Bump Go 1.26 → 1.26.1
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.
