core: Install first context as implicit default (and warn about its use)

There was a behaviour change in libusb 1.0.25 which triggers issues when
the API is misused. This caused for instance gutenprint to crash, see

This seems to affect several applications "out in the wild".

For now, work around this by installing an implicit default. But, change
the code to log an error in case this "feature" is being used.

This will allow some grace time for developers to fix their
applications, before we at a later point revert to the stricter

Fixes #1089
4 files changed