Compile on Windows

[cretz]

This cannot currently compile on Windows because #267 incorporated github.com/u-root/u-root/pkg/rand, which doesn't have a Windows version at https://github.com/u-root/u-root/tree/master/pkg/rand causing:

vendor\github.com\insomniacslk\dhcp\dhcpv6\dhcpv6message.go:10:2: build constraints exclude all Go files in c:\redacted\vendor\github.com\u-root\u-root\pkg\rand

[jhawk28]

#279 would fix this

[pmazzini]

Based on the below PRs, I think this should be fixed.

• Use Go crypto/rand as rand implementation for Windows u-root/u-root#2019
• Make server4 package compile on Windows #428
• Make server6 package compile on Windows #431