Do not obliterate errno value in the main thread during ptrace(2) call on x86.

Since ptrace(2) syscall can return -1 for non-error situations, libc
wrappers set errno to 0 before performing the syscall, as the service
to the caller.  On both i386 and amd64, the errno symbol was directly
referenced, which only works correctly in single-threaded process.

Change assembler wrappers for ptrace(2) to get current thread errno
location by calling __error().  Allow __error interposing, as
currently allowed in cerror().

Sponsored by:	The FreeBSD Foundation
MFC after:	1 week

lib/libc/amd64/sys/ptrace.S

@@ -38,14 +38,16 @@ __FBSDID("$FreeBSD$");
 
 #include "SYS.h"
 
+	.globl	CNAME(__error)
+	.type	CNAME(__error),@function
+
 ENTRY(ptrace)
-	xorl	%eax,%eax
 #ifdef PIC
-	movq	PIC_GOT(CNAME(errno)),%r8
-	movl	%eax,(%r8)
+	callq	PIC_PLT(CNAME(__error))
 #else
-	movl	%eax,CNAME(errno)(%rip)
+	callq	CNAME(__error)
 #endif
+	movl	$0,(%rax)
 	mov	$SYS_ptrace,%eax
 	KERNCALL
 	jb	HIDENAME(cerror)

lib/libc/i386/sys/ptrace.S

@@ -38,16 +38,18 @@ __FBSDID("$FreeBSD$");
 
 #include "SYS.h"
 
+	.globl	CNAME(__error)
+	.type	CNAME(__error),@function
+
 ENTRY(ptrace)
-	xorl	%eax,%eax
 #ifdef PIC
 	PIC_PROLOGUE
-        movl    PIC_GOT(CNAME(errno)),%edx
-        movl    %eax,(%edx)
+	call	PIC_PLT(CNAME(__error))
 	PIC_EPILOGUE
 #else
-        movl    %eax,CNAME(errno)
+	call	CNAME(__error)
 #endif
+	movl	$0,(%eax)
 	mov	$SYS_ptrace,%eax
 	KERNCALL
 	jb	HIDENAME(cerror)

lib/libc/sys/ptrace.2

@@ -2,7 +2,7 @@
 .\"	$NetBSD: ptrace.2,v 1.2 1995/02/27 12:35:37 cgd Exp $
 .\"
 .\" This file is in the public domain.
-.Dd July 28, 2016
+.Dd August 28, 2016
 .Dt PTRACE 2
 .Os
 .Sh NAME
@@ -906,7 +906,13 @@ to return
 \-1
 as a non-error value; to disambiguate,
 .Va errno
-can be set to 0 before the call and checked afterwards.
+is set to 0 in the libc wrapper for the
+.Fn ptrace
+system call and
+.Fn ptrace
+callers can reliably check
+.Va errno
+for non-zero value afterwards.
 .Sh ERRORS
 The
 .Fn ptrace