Perform signature verifiction when generating addresses.

This change adds an additional check when creating a new wallet or
extending the keypool.  All public and private keypairs are parsed
from their serialized forms, and an ecdsa signature is created and
verified using the keypairs.  If the verifiction fails at any point,
the wallet creation or keypool extension is aborted to prevent any
errors where an address is returned to a user, but any funds send to
that address are unspendable due to a mismatched keypair.