Fix multiple nss_compat initgroups() bugs

Fix multiple nss_compat initgroups() bugs

Compat initgroups() is completely broken; the code will always set
skip_initgroups_dyn to true, so initgroups() will never be actually
called, but due to the nature of the code, setgrent() won't be called
either - thus, subsequent invocations of initgroups() will not return
the NIS group list anymore.

This is a simple patch that makes sure skip_initgroups_dyn is set only
in case initgroups is not available; it also attempts to handle the
unavailability of other NSS interfaces better.

Conflicts:

	ChangeLog
(cherry picked from commit 3056dcdb783a177158a5617657d1a33f872358d6)