sendmany from all accounts

Without this change `sendmany` requires a specific account to send coins from. So we cannot send coins the same way `sendtoaddress` and the GUI do. The latter actually send coin from the `""` account but when they check the balance they get the whole wallet balance, not only the `""` balance.

This commit implements the same behavior in the sendmany command when the account name `"*"` is provided.

This will help fix the negative balance problem with Peershares.