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/syscall_linux.go

@@ -11,7 +11,10 @@
 
 package unix
 
-import "unsafe"
+import (
+	"syscall"
+	"unsafe"
+)
 
 /*
  * Wrapped
@@ -194,18 +197,18 @@ func (w WaitStatus) ExitStatus() int {
 	return int(w>>shift) & 0xFF
 }
 
-func (w WaitStatus) Signal() Signal {
+func (w WaitStatus) Signal() syscall.Signal {
 	if !w.Signaled() {
 		return -1
 	}
-	return Signal(w & mask)
+	return syscall.Signal(w & mask)
 }
 
-func (w WaitStatus) StopSignal() Signal {
+func (w WaitStatus) StopSignal() syscall.Signal {
 	if !w.Stopped() {
 		return -1
 	}
-	return Signal(w>>shift) & 0xFF
+	return syscall.Signal(w>>shift) & 0xFF
 }
 
 func (w WaitStatus) TrapCause() int {
@@ -818,7 +821,7 @@ func Mount(source string, target string, fstype string, flags uintptr, data stri
 //sysnb	InotifyInit() (fd int, err error)
 //sysnb	InotifyInit1(flags int) (fd int, err error)
 //sysnb	InotifyRmWatch(fd int, watchdesc uint32) (success int, err error)
-//sysnb	Kill(pid int, sig Signal) (err error)
+//sysnb	Kill(pid int, sig syscall.Signal) (err error)
 //sys	Klogctl(typ int, buf []byte) (n int, err error) = SYS_SYSLOG
 //sys	Link(oldpath string, newpath string) (err error)
 //sys	Listxattr(path string, dest []byte) (sz int, err error)
@@ -861,7 +864,7 @@ func Setgid(uid int) (err error) {
 //sys	Sync()
 //sysnb	Sysinfo(info *Sysinfo_t) (err error)
 //sys	Tee(rfd int, wfd int, len int, flags int) (n int64, err error)
-//sysnb	Tgkill(tgid int, tid int, sig Signal) (err error)
+//sysnb	Tgkill(tgid int, tid int, sig syscall.Signal) (err error)
 //sysnb	Times(tms *Tms) (ticks uintptr, err error)
 //sysnb	Umask(mask int) (oldmask int)
 //sysnb	Uname(buf *Utsname) (err error)