admin: Move admin_server.{h,c} to admin.{h,c}

admin: Move admin_server.{h,c} to admin.{h,c}

This change is merely because admin_server would contain all the code
from dispatchers and helpers to the actual APIs. Admin should have
similar structure to the daemon-side remote driver - dispatchers and
helpers in a separate module, APIs in a separate module.

Best viewed with -M.

Signed-off-by: Erik Skultety <eskultet@redhat.com>
Signed-off-by: Martin Kletzander <mkletzan@redhat.com>