Use madvise syscall wrapper from golang.org/x/sys/unix

Direct syscalls using syscall.Syscall(SYS_*, ...) should no longer be
used on darwin, see [1]. Instead, use the madvise syscall wrapper
provided by the golang.org/x/sys/unix package for all unix platforms.
This implement the same functionality.

[1] https://golang.org/doc/go1.12#darwin

As suggested by @ptabor in etcd-io/etcd#12316 (comment)

.travis.yml

@@ -7,6 +7,7 @@ go:
 - 1.12
 
 before_install:
+- go get -v golang.org/x/sys/unix
 - go get -v honnef.co/go/tools/...
 - go get -v github.com/kisielk/errcheck
 

bolt_unix.go

@@ -7,6 +7,8 @@ import (
 	"syscall"
 	"time"
 	"unsafe"
+
+	"golang.org/x/sys/unix"
 )
 
 // flock acquires an advisory lock on a file descriptor.
@@ -55,7 +57,7 @@ func mmap(db *DB, sz int) error {
 	}
 
 	// Advise the kernel that the mmap is accessed randomly.
-	err = madvise(b, syscall.MADV_RANDOM)
+	err = unix.Madvise(b, syscall.MADV_RANDOM)
 	if err != nil && err != syscall.ENOSYS {
 		// Ignore not implemented error in kernel because it still works.
 		return fmt.Errorf("madvise: %s", err)
@@ -82,12 +84,3 @@ func munmap(db *DB) error {
 	db.datasz = 0
 	return err
 }
-
-// NOTE: This function is copied from stdlib because it is not available on darwin.
-func madvise(b []byte, advice int) (err error) {
-	_, _, e1 := syscall.Syscall(syscall.SYS_MADVISE, uintptr(unsafe.Pointer(&b[0])), uintptr(len(b)), uintptr(advice))
-	if e1 != 0 {
-		err = e1
-	}
-	return
-}

go.mod

@@ -2,4 +2,4 @@ module go.etcd.io/bbolt
 
 go 1.12
 
-require golang.org/x/sys v0.0.0-20200202164722-d101bd2416d5
+require golang.org/x/sys v0.0.0-20200923182605-d9f96fdee20d

go.sum

@@ -1,2 +1,2 @@
-golang.org/x/sys v0.0.0-20200202164722-d101bd2416d5 h1:LfCXLvNmTYH9kEmVgqbnsWfruoXZIrh4YBgqVHtDvw0=
-golang.org/x/sys v0.0.0-20200202164722-d101bd2416d5/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
+golang.org/x/sys v0.0.0-20200923182605-d9f96fdee20d h1:L/IKR6COd7ubZrs2oTnTi73IhgqJ71c9s80WsQnh0Es=
+golang.org/x/sys v0.0.0-20200923182605-d9f96fdee20d/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=