unrealircd::rpc_password is now encrypted with secret::key (XChaCha20-Poly1305-IETF)

unrealircd::rpc_password is now encrypted with secret::key (XChaCha20-Poly1305-IETF)
Again, the purpose is so if any bad person gets a copy of your DB then the
stored RPC password is still useless since they also need your config/config.php.

Old unencrypted unrealircd::rpc_password entries are automatically
encrypted (upgraded).

Similar to previous commit 6b08fcb99e66665e7e4f345702915d7192fcd27b
this means you cannot blindly 'rm config/config.php' and then expect
your existing DB to still work with a random new (and different) key.