pktsock: use math.Rand over crypto.Rand

This prevents blocking until the CRNG is initialized with Linux ≥ 4.16:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=43838a23a05fbd13e47d750d3dfd77001536dd33

Randomness is used to fill the IPv4 ID field. As per
https://tools.ietf.org/html/rfc6864, the ID field is used only for
fragmentation, but DHCP packets are never fragmented. Even if they were, the
randomness does not need to be cryptographically secure.

pktsock_linux.go

@@ -1,8 +1,8 @@
 package dhcp4client
 
 import (
-	"crypto/rand"
 	"encoding/binary"
+	"math/rand"
 	"net"
 	"time"