Remove github.com/pkg/errors and replace with std library version

This is possible now that we no longer support go1.12 and brings
rclone into line with standard practices in the Go world.

This also removes errors.New and errors.Errorf from lib/errors and
prefers the stdlib errors package over lib/errors.

lib/mmap/mmap_unix.go

@@ -7,7 +7,8 @@
 package mmap
 
 import (
-	"github.com/pkg/errors"
+	"fmt"
+
 	"golang.org/x/sys/unix"
 )
 
@@ -17,7 +18,7 @@ import (
 func Alloc(size int) ([]byte, error) {
 	mem, err := unix.Mmap(-1, 0, size, unix.PROT_READ|unix.PROT_WRITE, unix.MAP_PRIVATE|unix.MAP_ANON)
 	if err != nil {
-		return nil, errors.Wrap(err, "mmap: failed to allocate memory for buffer")
+		return nil, fmt.Errorf("mmap: failed to allocate memory for buffer: %w", err)
 	}
 	return mem, nil
 }
@@ -28,7 +29,7 @@ func Alloc(size int) ([]byte, error) {
 func Free(mem []byte) error {
 	err := unix.Munmap(mem)
 	if err != nil {
-		return errors.Wrap(err, "mmap: failed to unmap memory")
+		return fmt.Errorf("mmap: failed to unmap memory: %w", err)
 	}
 	return nil
 }