hd-seed: proper version bytes for testnet

Please don't merge yet. Contains questionable solutions and patchy code.

Currently, `sx-seed` will always generate extended private/public keys with xpriv/xpub prefix, regardless of whether testnet is used. [This is wrong](https://github.com/bitcoin/bips/blob/master/bip-0032.mediawiki#serialization-format).

The `hd_private_key` prototypes in `libwallet` have a way to do that by specifying a boolean flag, or explicitly setting lineage.

However, `sx` itself is network-agnostic, and there is no simple way to determine which version bytes have to be used.

Rather than making sx coupled with a network, we could use a setting in `sx.cfg` - "testnet" for now, perhaps rename to "network" when it becomes relevant.

Note how this adds an ugly singled-out library in `Makefile.am`, which falls out of the general syntax used in the build file.