Complete skeleton redesign

 * Copied tcp.c from conntrack-tools.
 * Delete master/slave approach.
 * Complete the CLI argument parsing to config.
 * Add a basic CMD interface.

By now, nft-syncd is able to:

 * Open a netlink socket and wait for events using libevent.
 * Open a TCP socket and wait for connections (not with libevent yet).
 * Connect to other client (not with libevent yet).

By now, two instances of nft-syncd can be easily connected this way:

 ~:# nft-syncd -l 127.0.0.1 -c otherLocalIP -p 10001 &
 ~:# nft-syncd -l otherLocalIP -c 127.0.0.1 -p 10001

Note this connections does nothing ATM. Just connect.

More work in near future.