all: Fix some linter errors (#5499)

I'm working through linter complaints, these are some fixes. Broad
categories:

1) Ignore errors where we can ignore errors: add "_ = ..." construct.
you can argue that this is annoying noise, but apart from silencing the
linter it *does* serve the purpose of highlighting that an error is
being ignored. I think this is OK, because the linter highlighted some
error cases I wasn't aware of (starting CPU profiles, for example).

2) Untyped constants where we though we had set the type.

3) A real bug where we ineffectually assigned to a shadowed err.

4) Some dead code removed.

There'll be more of these, because not all packages are fixed, but the
diff was already large enough.

cmd/syncthing/gui_auth.go

@@ -20,7 +20,7 @@ import (
 	"github.com/syncthing/syncthing/lib/rand"
 	"github.com/syncthing/syncthing/lib/sync"
 	"golang.org/x/crypto/bcrypt"
-	"gopkg.in/ldap.v2"
+	ldap "gopkg.in/ldap.v2"
 )
 
 var (
@@ -80,11 +80,10 @@ func basicAuthAndSessionMiddleware(cookieName string, guiCfg config.GUIConfigura
 			return
 		}
 
-		authOk := false
 		username := string(fields[0])
 		password := string(fields[1])
 
-		authOk = auth(username, password, guiCfg, ldapCfg)
+		authOk := auth(username, password, guiCfg, ldapCfg)
 		if !authOk {
 			usernameIso := string(iso88591ToUTF8([]byte(username)))
 			passwordIso := string(iso88591ToUTF8([]byte(password)))