clean-up "nolint" comments, remove unused ones

- fix "nolint" comments to be in the correct format (`//nolint:<linters>[,<linter>`
  no leading space, required colon (`:`) and linters.
- remove "nolint" comments for errcheck, which is disabled in our config.
- remove "nolint" comments that were no longer needed (nolintlint).
- where known, add a comment describing why a "nolint" was applied.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

integration/client/restart_monitor_test.go

@@ -40,8 +40,7 @@ import (
 	exec "golang.org/x/sys/execabs"
 )
 
-// the following nolint is for shutting up gometalinter on non-linux.
-// nolint: unused
+//nolint:unused // Ignore on non-Linux
 func newDaemonWithConfig(t *testing.T, configTOML string) (*Client, *daemon, func()) {
 	if testing.Short() {
 		t.Skip()