unix: add ErrnoName and SignalName

Add ErrnoName and SignalName to get errno and signal name strings from
syscall.Errno and syscall.Signal values, respectively.

This repurposes the errors and signals vars (because they are not used
within x/sys/unix currently) and turns them into slices of struct,
containing errno/signal number, name and description. ErrnoName and
SignalName can then be trivially implemented using sort.Search.

Renaming errors to errorList additionaly allows to avoid package aliases
for the errors package.

Fixes golang/go#25134

Change-Id: Ie195872793f44c437f0f175ccfaa13a2546338c5
Reviewed-on: https://go-review.googlesource.com/110875
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/cap_freebsd.go

@@ -7,7 +7,7 @@
 package unix
 
 import (
-	errorspkg "errors"
+	"errors"
 	"fmt"
 )
 
@@ -60,26 +60,26 @@ func CapRightsSet(rights *CapRights, setrights []uint64) error {
 
 	n := caparsize(rights)
 	if n < capArSizeMin || n > capArSizeMax {
-		return errorspkg.New("bad rights size")
+		return errors.New("bad rights size")
 	}
 
 	for _, right := range setrights {
 		if caprver(right) != CAP_RIGHTS_VERSION_00 {
-			return errorspkg.New("bad right version")
+			return errors.New("bad right version")
 		}
 		i, err := rightToIndex(right)
 		if err != nil {
 			return err
 		}
 		if i >= n {
-			return errorspkg.New("index overflow")
+			return errors.New("index overflow")
 		}
 		if capidxbit(rights.Rights[i]) != capidxbit(right) {
-			return errorspkg.New("index mismatch")
+			return errors.New("index mismatch")
 		}
 		rights.Rights[i] |= right
 		if capidxbit(rights.Rights[i]) != capidxbit(right) {
-			return errorspkg.New("index mismatch (after assign)")
+			return errors.New("index mismatch (after assign)")
 		}
 	}
 
@@ -95,26 +95,26 @@ func CapRightsClear(rights *CapRights, clearrights []uint64) error {
 
 	n := caparsize(rights)
 	if n < capArSizeMin || n > capArSizeMax {
-		return errorspkg.New("bad rights size")
+		return errors.New("bad rights size")
 	}
 
 	for _, right := range clearrights {
 		if caprver(right) != CAP_RIGHTS_VERSION_00 {
-			return errorspkg.New("bad right version")
+			return errors.New("bad right version")
 		}
 		i, err := rightToIndex(right)
 		if err != nil {
 			return err
 		}
 		if i >= n {
-			return errorspkg.New("index overflow")
+			return errors.New("index overflow")
 		}
 		if capidxbit(rights.Rights[i]) != capidxbit(right) {
-			return errorspkg.New("index mismatch")
+			return errors.New("index mismatch")
 		}
 		rights.Rights[i] &= ^(right & 0x01FFFFFFFFFFFFFF)
 		if capidxbit(rights.Rights[i]) != capidxbit(right) {
-			return errorspkg.New("index mismatch (after assign)")
+			return errors.New("index mismatch (after assign)")
 		}
 	}
 
@@ -130,22 +130,22 @@ func CapRightsIsSet(rights *CapRights, setrights []uint64) (bool, error) {
 
 	n := caparsize(rights)
 	if n < capArSizeMin || n > capArSizeMax {
-		return false, errorspkg.New("bad rights size")
+		return false, errors.New("bad rights size")
 	}
 
 	for _, right := range setrights {
 		if caprver(right) != CAP_RIGHTS_VERSION_00 {
-			return false, errorspkg.New("bad right version")
+			return false, errors.New("bad right version")
 		}
 		i, err := rightToIndex(right)
 		if err != nil {
 			return false, err
 		}
 		if i >= n {
-			return false, errorspkg.New("index overflow")
+			return false, errors.New("index overflow")
 		}
 		if capidxbit(rights.Rights[i]) != capidxbit(right) {
-			return false, errorspkg.New("index mismatch")
+			return false, errors.New("index mismatch")
 		}
 		if (rights.Rights[i] & right) != right {
 			return false, nil