Every GTask should have a name! This helps when debugging problems.

Also, for every GTask used in an async operation, we should check (a)
that the source tag matches, and (b) that the task is valid. This uses
g_return assertions for anything applications can screw up, and g_assert
for anything internal to glib-networking.

Lastly, there's no need to set a source tag in
g_libproxy_resolver_lookup() since that's a sync function and there's
nowhere it can ever be checked.