Rename "curve25519" to "salsa20-poly1305"

[DarkKirb]

The documentation currently refers to "curve25519" as an encryption algorithm, which is something it can't do.

Curve25519 is an ECDH curve which is used to generate an encryption key. Curve25519 is independent from the actual encryption method used.

I propose that we rename the "curve25519" encryption to "salsa20-poly1305" which is the encryption method used by rdedup currently. This would also allow additional encryption algorithms to be named sensibly, like for example "chacha20-poly1305", "aes256-gcm", etc.

[dpc]

:+1: with some backward compat. handling + warning when the old name is used.