Does bitcoind (as of 0.15) support BIP39 mnemonic phrase for wallets? Can bitcoind print out a BIP39 seed phrase to backup the wallet?

Can I import a BIP39 seed phrase (from, for example, Breadwallet on iOS)into bitcoind?

I know bitcoind wallets have been BIP32 format since 0.13, what is used as the seed? What is backed up when I make a copy of wallet.dat? If the wallet is encrypted, does the seed change (the way BIP32 uses a passphrase as "the 13th word"?)
No, bitcoind does not support BIP39 seed phrase backup. It supports deterministic key derivation (BIP32), and the seed is just random number generated on wallet creation. Moreover, the generated keys do not follow BIP44 key paths, so even if you imported successfully in some way your seed to bitcoind wallet, there is no chance to see your money from wallets which follow BIP44 key paths.

As far I know wallet encryption in bitcoind has more to do with symmetric encryption of private key data. It has nothing to do with seed converted to seed phrases.