ipa_range_check: Use special attributes to determine presence of RID bases

The slapi_entry_attr_get_ulong which is used to get value of the RID base
attributes returns 0 in case the attribute is not set at all. We need
to distinguish this situation from the situation where RID base attributes
are present, but deliberately set to 0.

Otherwise this can cause false negative results of checks in the range_check
plugin.

Part of: https://fedorahosted.org/freeipa/ticket/4137
ipa_range_check: Connect the new node of the linked list

Part of: https://fedorahosted.org/freeipa/ticket/4137
ipa_range_check: Make a new copy of forest_root_id attribute for range_info struct

Not making a new copy of this attribute creates multiple frees caused by multiple
pointers to the same forest_root_id from all the range_info structs for all the
domains belonging to given forest.

Part of: https://fedorahosted.org/freeipa/ticket/4137
ipa_range_check: Do not fail when no trusted domain is available

When building the domain to forest root map, we need to take the case
of IPA server having no trusted domains configured at all. Do not abort
the checks, but return an empty map instead.

Part of: https://fedorahosted.org/freeipa/ticket/4137
ipa_range_check: Fix typo when comparing strings using strcasecmp

Part of: https://fedorahosted.org/freeipa/ticket/4137
ipatests: Extend test suite for ID ranges

Add tests coverage for recently added ID range checks dependant
on the ID range types.

Part of: https://fedorahosted.org/freeipa/ticket/4137
ipatests: Change range_check return values from int to range_check_result_t enum

Using integers for return values that are used for complex casing can be fragile
and typo-prone. Change range_check function to return range_check_result_t enum,
whose values properly describes each of the range_check results.

Part of: https://fedorahosted.org/freeipa/ticket/4137
Merge branch 'extend_ipa_range_check2' into test