unix: split Pipe/Pipe2/Dup2, use *at replacement calls

This brings over three CLs from the main syscall package:
http://golang.org/cl/5833
http://golang.org/cl/5835
http://golang.org/cl/5837

Pipe, Pipe2, and Dup2 are moved from syscall_linux.go to the GOARCH
specific variants.  On 386 and amd64, Linux kernel version 2.6.23 (the
documented minimum Linux kernel version the Go distribution supports)
does not support the pipe2 system call, so Pipe continues to call
pipe.  On ARM, Pipe now calls pipe2.

Several system calls are reimplemented in terms of the *at syscalls,
passing AT_FDCWD to indicate that pathnames are to be interpreted
relative to the current directory.  The *at syscalls were added in
Linux kernel version 2.6.16.

This is in preparation for arm64 support, as the arm64 Linux kernel
does not provide the traditional syscall variants.

Change-Id: Id6bc6097dc5f4324cd9e429c5e1f3a411a08ce42
Reviewed-on: https://go-review.googlesource.com/10032
Reviewed-by: Rob Pike <r@golang.org>

unix/syscall_linux_amd64.go

@@ -6,7 +6,7 @@ package unix
 
 import "syscall"
 
-//sys	Chown(path string, uid int, gid int) (err error)
+//sys	Dup2(oldfd int, newfd int) (err error)
 //sys	Fadvise(fd int, offset int64, length int64, advice int) (err error) = SYS_FADVISE64
 //sys	Fchown(fd int, uid int, gid int) (err error)
 //sys	Fstat(fd int, stat *Stat_t) (err error)
@@ -100,6 +100,32 @@ func NsecToTimeval(nsec int64) (tv Timeval) {
 	return
 }
 
+//sysnb	pipe(p *[2]_C_int) (err error)
+
+func Pipe(p []int) (err error) {
+	if len(p) != 2 {
+		return EINVAL
+	}
+	var pp [2]_C_int
+	err = pipe(&pp)
+	p[0] = int(pp[0])
+	p[1] = int(pp[1])
+	return
+}
+
+//sysnb pipe2(p *[2]_C_int, flags int) (err error)
+
+func Pipe2(p []int, flags int) (err error) {
+	if len(p) != 2 {
+		return EINVAL
+	}
+	var pp [2]_C_int
+	err = pipe2(&pp, flags)
+	p[0] = int(pp[0])
+	p[1] = int(pp[1])
+	return
+}
+
 func (r *PtraceRegs) PC() uint64 { return r.Rip }
 
 func (r *PtraceRegs) SetPC(pc uint64) { r.Rip = pc }