GitLab

- use unsigned int in for loops
- NULL pointer rather than NULL in function descriptions

New test check that it is possible to specify a
non-standard option within a host declaration.

These tests check that it is possible to specify options
for hosts.

Corrected mixed up use of DHCPv4 vs DHCPv6 and the
dhcp4 vs dhcp6 option space names.

Addressed a couple of minor issues and added unit tests for
switching between various lease database backends.