Mark all the methods on the device as async, so we
can get access to the associated DBusGMethodInvocation.
When claiming the device, remember the sender, and for every
API entry point, check that the sender is the same as the one
that made the original claim.
Trying to enroll a user whilst the device is already claimed
from another program will fail with:
** ERROR **: failed to claim device: Device was already claimed
This is the first step towards PolicyKit and multi-user support
Make all of FPrintManager's struct members private,
warn of errors using a _get_error() function, as object
creation can never fail.
The only error possible shouldn't really be an error though,
as it only means that no devices will be enumerated, but it
could tell us about newly plugged devices instead.
