Add structcheck, unused, and varcheck linters.

Warn on unused and dead code

Signed-off-by: Daniel Nephin <dnephin@gmail.com>

debug/debug.go

@@ -4,7 +4,6 @@ import (
 	"bufio"
 	"fmt"
 	"os"
-	"sort"
 	"strconv"
 	"strings"
 )
@@ -55,12 +54,3 @@ func getMaps(pid int) (map[string]int, error) {
 	}
 	return smaps, nil
 }
-
-func keys(smaps map[string]int) []string {
-	var o []string
-	for k := range smaps {
-		o = append(o, k)
-	}
-	sort.Strings(o)
-	return o
-}