Inconsistent requirement for requiredMajor

[joshuahannan]

In WalletAdminLib's checkRequiredChange function, a check is performed to ensure the number of required signatures in a major transaction is at least two less than the number of owners. However, this is never enforced in WalletMainLib's init function, which only requires _requiredMajor to be smaller or equal than the number of owners.
Consider abstracting these requirements into a common function, so as to have consistent behavior in initialization and updating.