More mail API churn... reversing some previous API decisions.

I've made some key API changes to EMailSession on the account-mgmt
branch which should allow for this, and will hopefully also benefit
the "email-factory" branch.

EMailBackend barely needs to exist anymore, except as the owner of
EMailSession.

For several low-level functions, we replace its EMailBackend parameter
with EMailSession and EAlertSink parameters; the latter so it can still
pass user alerts up the chain.