Add registername RPC with shared name_new commitment helper

[mstrofnone]

Add a new registername RPC method that provides a simple single-call interface for name registration:

registername "myname" "my-value"

Unlike name_firstupdate (which requires a prior name_new and takes rand/tx/value as separate positional args), registername takes just a name and value, handling the commitment automatically:

1. Scan the wallet for a matching name_new commitment.
2. If none is found, create one automatically via name_new.
3. Build the name_firstupdate transaction.
4. If mempool-eligible, broadcast; otherwise commit to wallet for automatic rebroadcast after the name_new matures.

Refactoring

Factors out the wallet scanning logic from name_firstupdate into a shared findNameNewCommitment() helper that is reused by both name_firstupdate and registername. This eliminates duplicated wallet scan code and makes the commitment-finding logic testable independently.

The core logic for creating a name_new and building a name_firstupdate is not called via the RPC method name_new() with all the RPC boilerplate around it, but instead uses the factored-out helper functions directly.

name_firstupdate is completely unchanged in behavior — power users can still do the two-step manual procedure.

Returns {txid, rand} as a JSON object.

Tests

Includes functional tests (name_registername.py) covering:

• Basic registration with auto name_new
• Reuse of existing name_new commitment
• Duplicate name rejection
• Default (empty) value
• Parameter validation (name too long, value too long)
• Return format verification

Ref: namecoin#576, namecoin#579, namecoin#581