switch from package syscall to x/sys/unix

The syscall package is locked down and the comment in [1] advises to
switch code to use the corresponding package from golang.org/x/sys. Do
so and replace usage of package syscall with package
golang.org/x/sys/unix where applicable.

  [1] https://github.com/golang/go/blob/master/src/syscall/syscall.go#L21-L24

This will also allow to get updates and fixes for syscall wrappers
without having to use a new go version.

Errno, Signal and SysProcAttr aren't changed as they haven't been
implemented in /x/sys/. Stat_t from syscall is used if standard library
packages (e.g. os) require it. syscall.SIGTERM is used for
cross-platform files.

pkg/util/rlimit/rlimit_linux.go

@@ -19,9 +19,9 @@ limitations under the License.
 package rlimit
 
 import (
-	"syscall"
+	"golang.org/x/sys/unix"
 )
 
 func RlimitNumFiles(maxOpenFiles uint64) {
-	syscall.Setrlimit(syscall.RLIMIT_NOFILE, &syscall.Rlimit{Max: maxOpenFiles, Cur: maxOpenFiles})
+	unix.Setrlimit(unix.RLIMIT_NOFILE, &unix.Rlimit{Max: maxOpenFiles, Cur: maxOpenFiles})
 }