Add support for funding z_sendmany from a UA account

[str4d]

z_sendmany won't take an account number as the first argument (that would complicate the RPC UX too much); instead allow it to take a UA as the fromaddress.

• Errors if fromaddress is a UA that does not correspond to any existing account in the wallet.
• If a UA with multiple components is provided as the fromaddress, take funds only from one pool; the most recent one that has enough funds (if any).
• Add an argument flag that indicates that no amounts should be revealed as part of the transfer.
  • Errors if e.g. user specifies recipients from multiple pools.
• Change is handled as-currently, i.e. returned to the first involved address from which the funds are taken.
  • This means that if a UA has Sapling and Orchard, with sufficient funds in Sapling, the change goes back to the Sapling receiver.

[nuttycom]

This will be resolved by #5469

[str4d]

Closed by #5469, merged into #5419.