chore(pmtud): remove calls to syscall in favor of unix and windows

- syscall is deprecated and is not kept up-to-date
- each OS is inherently different hence the syscall being deprecated

internal/pmtud/icmp/df_linux.go

@@ -1,10 +1,10 @@
 package icmp
 
 import (
-	"syscall"
+	"golang.org/x/sys/unix"
 )
 
 func setDontFragment(fd uintptr) (err error) {
-	return syscall.SetsockoptInt(int(fd), syscall.IPPROTO_IP,
-		syscall.IP_MTU_DISCOVER, syscall.IP_PMTUDISC_PROBE)
+	return unix.SetsockoptInt(int(fd), unix.IPPROTO_IP,
+		unix.IP_MTU_DISCOVER, unix.IP_PMTUDISC_PROBE)
 }