src: add masquerade support

src: add masquerade support

This patch adds masquerade support for nft.

The syntax is:

 % nft add rule nat postrouting masquerade [flags]

Currently, flags are:
 random, random-fully, persistent

Example:
 % nft add rule nat postrouting masquerade random,persistent

Signed-off-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>