Fix: Warn on unused linked list function call return values
Fix: check return value of ll_insert()
Fix: Check return value of ll_delete()
Fix: Warn on unused result for avl function calls
Fix: Warn on unused result for internal avl function calls
Fix: Warn on unused result for hashtable function calls
Fix: Check return value of ht_init() in tests
Merge pull request #242 from matthiasbeyer/add-warn_unused_result

Add __warn_unused_result__
Use errno.h return constants on insert operation
Change: set interface/impl. to return status on r_set_remove()
Use errno.h constants on delete operation
Use errno constants on destroy operations
Added note which errno const is returned

This patch just changes the interface definition for the not implemented
functions.

Currently, only -ENOMEM is added in the list of possibly returned errno
constants.
Merge pull request #247 from matthiasbeyer/change-to_errno

Change function interfaces and implementations to use errno.h constants
Hotfix: failing ll tests
Merge pull request #248 from matthiasbeyer/hotfix-ll_tests

Hotfix: ll tests on master failing
Add: Implementation for ll_union()
Outsourced set type into own header file
Add: Implementation for r_set_union()
Added warning in set.h for partially done union operation in case of error