What is the default relaying policy for RBF double spend transactions? Let's say I broadcast a transation tx0 signaling RBF replaceability. Later on, I realize it has a small amount of fees, so I send another transaction tx1 that spends the same inputs than tx0, but this time with higher fees. 

Which is the default bitcoind/bitcoin-qt behaviour regarding the relaying of tx1? Would all nodes broadcast the new version of the transaction or does the relay policy depend on the mempoolreplacement parameter? Do any special rules apply to the relaying of the replacement transaction?

I have seen that Peter Todd's RBF proposal included a NODE_REPLACE_BY_FEE service bit. Is anything like this included in the current version?
Nodes will only relay if mempoolreplacement hasn't set to false (most nodes probably do relay because the default is true).
There are serval rules for accepting a replacement after BIP125 (check the BIP for more details). In general: The replacement must pay greater fees than the transactions it replaces - if we did the bandwidth used by those conflicting transactions would not be paid for.