replace "dep" with "go mod"

A simple "go mod init && go mod tidy && go mod vendor" was enough and
mostly seems to have picked the same revisions of the dependencies as
before. Notable exceptions are klog and gogo/protobuf.

vendor/golang.org/x/sys/unix/syscall_unix.go

@@ -351,7 +351,11 @@ func SetsockoptLinger(fd, level, opt int, l *Linger) (err error) {
 }
 
 func SetsockoptString(fd, level, opt int, s string) (err error) {
-	return setsockopt(fd, level, opt, unsafe.Pointer(&[]byte(s)[0]), uintptr(len(s)))
+	var p unsafe.Pointer
+	if len(s) > 0 {
+		p = unsafe.Pointer(&[]byte(s)[0])
+	}
+	return setsockopt(fd, level, opt, p, uintptr(len(s)))
 }
 
 func SetsockoptTimeval(fd, level, opt int, tv *Timeval) (err error) {