Auto-enter slaveMode when SYNC from redis-cli

If someone asks for SYNC or PSYNC from redis-cli,
automatically enter slaveMode (as if they ran
redis-cli --slave) and continue printing the replication
stream until either they Ctrl-C or the master gets disconnected.
Merge pull request #1545 from mattsta/fix-redis-cli-sync

Deny SYNC and PSYNC in redis-cli