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_amd64.go

@@ -4,6 +4,8 @@
 
 package unix
 
+import "syscall"
+
 //sys	Chown(path string, uid int, gid int) (err error)
 //sys	Fchown(fd int, uid int, gid int) (err error)
 //sys	Fstat(fd int, stat *Stat_t) (err error)
@@ -58,7 +60,7 @@ package unix
 func Getpagesize() int { return 4096 }
 
 //go:noescape
-func gettimeofday(tv *Timeval) (err Errno)
+func gettimeofday(tv *Timeval) (err syscall.Errno)
 
 func Gettimeofday(tv *Timeval) (err error) {
 	errno := gettimeofday(tv)