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_dragonfly.go

@@ -156,7 +156,7 @@ func Getfsstat(buf []Statfs_t, flags int) (n int, err error) {
 //sysnb	Gettimeofday(tv *Timeval) (err error)
 //sysnb	Getuid() (uid int)
 //sys	Issetugid() (tainted bool)
-//sys	Kill(pid int, signum Signal) (err error)
+//sys	Kill(pid int, signum syscall.Signal) (err error)
 //sys	Kqueue() (fd int, err error)
 //sys	Lchown(path string, uid int, gid int) (err error)
 //sys	Link(path string, link string) (err error)