go.sys/unix: use syscall.Errno for errors

If we use a local type, it won't compare properly with errors from
the rest of the standard library. Errors are the one type from syscall
that propagates through the system, so it's important to have only
one type for them.

Ditto for syscall.Signal.

LGTM=dave
R=rsc, dave
CC=golang-codereviews
https://golang.org/cl/123490043

unix/exec_bsd.go

@@ -8,6 +8,7 @@ package unix
 
 import (
 	"runtime"
+	"syscall"
 	"unsafe"
 )
 
@@ -34,12 +35,12 @@ func runtime_AfterFork()
 // For the same reason compiler does not race instrument it.
 // The calls to RawSyscall are okay because they are assembly
 // functions that do not grow the stack.
-func forkAndExecInChild(argv0 *byte, argv, envv []*byte, chroot, dir *byte, attr *ProcAttr, sys *SysProcAttr, pipe int) (pid int, err Errno) {
+func forkAndExecInChild(argv0 *byte, argv, envv []*byte, chroot, dir *byte, attr *ProcAttr, sys *SysProcAttr, pipe int) (pid int, err syscall.Errno) {
 	// Declare all variables at top in case any
 	// declarations require heap allocation (e.g., err1).
 	var (
 		r1, r2 uintptr
-		err1   Errno
+		err1   syscall.Errno
 		nextfd int
 		i      int
 	)