Fixes Issue 668

Fixes Issue 577

K-9 has earned its Canine Good Citizen.

K-9 now respects the Android-global Background data preference, by
default.  However, there is an override, allowing K-9 to ignore the
global preference, by either always running around and synchronizing
in the background, or staying put and not synchronizing.

A user can now elect to turn off mail synchronization across all
accounts in either the K-9 Preferences page by setting Background
synchronization to Never, or leaving that value as "When 'Background
data' is checked" and deselecting the Android-global Background data
preference.