libsmbconf: add get_includes() and set_includes() to the API.

libsmbconf: add get_includes() and set_includes() to the API.

Includes have to get a special treatment, at least for registry.
Includes are not like other smbconf parameters: they are some kind
of metainformation. "include" has two effects when stated twice so
it can not be stored boldly into registry, since there can only be
one value named "include" in registry per key.

I will provide special handling for includes for the registry backend.

This patch provides the necessary methods in the smbconf API.

Michael
(This used to be commit e86eb375d9f83f73aeea0a16c8b43e2ef21a6e20)