We rely on the include path to find many of these headers. It
especially doesn't make sense to use #include "foo.h" for headers
in the top-level include directory.
Wheh netgroup_base is set we now do out own netgroup lookups using
LDAP. Previously, LDAP was queried directly to get a list of the
netgroups the user belongs to but other netgroups queries went
through innetgr(3). This makes it possible to use netgroups
in LDAP sudoers on systems that don't have an innetgr() function.
GitHub issue #251.