nsswitch: handle missing actions properly

[Florian Weimer]

* Siddhesh Poyarekar:

> The fix is OK but there ought to be a regression test that verifies
> that getgroups() returns the full list of supplementary groups.  I'm
> not sure if the container testing can handle this though.

We can test getgrouplist, which does not change the supplementary
groups, but exercises the same NSS code in the background (via
internal_getgrouplist that is also called by initgroups).

Thanks,
Florian
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