Deny SYNC and PSYNC in redis-cli

Running SYNC or PSYNC in the cli returns the
Redis replication stream as output, then upon encountering
the next command, redis-cli aborts because it's receiving
a replication stream instead of command output.

Trying to read the replication stream from redis-cli isn't useful (as it breaks the CLI and doesn't feed you ongoing output anyway).

If you want to view/debug a live replication stream, you should do `cat <(echo sync) - | nc localhost 6379` instead.