fix(initsystem): don't fail WinSCM start/stop/restart on benign slow-start warning

[vikramhh]

On a Windows target, WinSCM.StartService/StopService/RestartService run
Start-Service/Stop-Service/Restart-Service with only -ErrorAction Stop.
When a service stays in StartPending/StopPending for a moment (common on a cold
first start doing real init work), PowerShell emits a non-terminating warning:

WARNING: Waiting for service '<display> (<name>)' to start...

That warning goes to the warning stream, which lands on stderr under a
subprocess. rig treats any Windows stderr output as a failure
(cmd.ErrWroteStderr) unless AllowWinStderr is set, so a successful-but-slow
start is reported as command wrote output to stderr. The command exits 0 and
the service does reach Running — only the benign warning trips the failure.
(The executor already sets $ProgressPreference='SilentlyContinue', which
covers the progress stream but not the warning stream.)

Fix: add -WarningAction SilentlyContinue to the three cmdlets. This silences
exactly the benign progress warning while -ErrorAction Stop still turns a
genuine failure into a terminating (non-zero) error, so real error detection is
preserved. Per review on the issue, this uses -WarningAction only and does
not set AllowWinStderr, which would be a blunt instrument that greenlights
any stderr output and could mask unexpected diagnostics.

EnableService/DisableService use Set-Service, which doesn't wait on a
state transition and so doesn't emit this warning — left unchanged.

Existing winscm_test.go success cases now also assert the flag is present;
they fail on the pre-fix code and pass after. go vet/gofmt clean.

Fixes #404