Lint on pkg/* packages

- pkg/useragent
- pkg/units
- pkg/ulimit
- pkg/truncindex
- pkg/timeoutconn
- pkg/term
- pkg/tarsum
- pkg/tailfile
- pkg/systemd
- pkg/stringutils
- pkg/stringid
- pkg/streamformatter
- pkg/sockets
- pkg/signal
- pkg/proxy
- pkg/progressreader
- pkg/pools
- pkg/plugins
- pkg/pidfile
- pkg/parsers
- pkg/parsers/filters
- pkg/parsers/kernel
- pkg/parsers/operatingsystem

Signed-off-by: Vincent Demeester <vincent@sbr.pm>

useragent/useragent.go

@@ -3,14 +3,9 @@
 package useragent
 
 import (
-	"errors"
 	"strings"
 )
 
-var (
-	ErrNilRequest = errors.New("request cannot be nil")
-)
-
 // VersionInfo is used to model UserAgent versions.
 type VersionInfo struct {
 	Name    string
@@ -30,7 +25,7 @@ func (vi *VersionInfo) isValid() bool {
 	return true
 }
 
-// Convert versions to a string and append the string to the string base.
+// AppendVersions converts versions to a string and appends the string to the string base.
 //
 // Each VersionInfo will be converted to a string in the format of
 // "product/version", where the "product" is get from the name field, while