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: tests for simple delete operation
Added test for destroying multiple nodes
Fix: return negative error number here
Fix: Check before regen_metadata() if there is a root node left