rpc: add registername with auto-commitment support

[mstrofnone]

Add a new registername RPC method that wraps name_firstupdate with improved UX: if no matching name_new commitment is found in the wallet, one is created automatically before proceeding.

Motivation

As discussed in namecoin#576 and namecoin#579, the separate name_autoregister RPC creates UX friction by being its own method. This PR rolls that functionality into registername (the Bitcoin Core convention alias for name_firstupdate), giving average users a single-call registration flow while preserving the two-step manual procedure for power users.

Behaviour

When called as registername:

1. Scan the wallet for a matching name_new commitment input.
2. If not found, call name_new automatically to create one.
3. Create the name_firstupdate transaction.
4. If the transaction is mempool-eligible, broadcast it; otherwise commit it to the wallet for automatic rebroadcast after the name_new matures (via the wallet's blockConnected rebroadcast logic).

When called as name_firstupdate (the original method), the old behaviour is preserved — it simply fails if no commitment is found.

Implementation

• name_firstupdate_impl() is a shared static helper parameterised by method name.
• name_firstupdate() and registername() both delegate to it.
• The auto-commitment path builds a fresh JSONRPCRequest for name_new reusing the caller's context.
• A wallet-transaction fallback is added to getNamePrevout for when the just-created name_new output is not yet in CoinsTip (unconfirmed).
• registername is registered as a separate RPC command (not an alias sharing unique_id) so it gets its own help text describing the auto-commitment behaviour.

Usage

# Simple one-call registration (auto-creates name_new if needed):
namecoin-cli registername "myname" "" "" "my-value"

# Power users can still do the two-step flow:
namecoin-cli name_new "myname"
# ... wait for confirmations ...
namecoin-cli name_firstupdate "myname" "rand" "txid" "my-value"

Ref: namecoin#576, namecoin#579