libsmbconf: add smbconf_create_set_share

libsmbconf: add smbconf_create_set_share

This call creates a new share definition, using the parameters provided
with a smbconf_service structure.
Such an interface allows for simple cloning of services with:
smbconf_get_share(conf_ctx, mem_ctx, base_sharename, &base_service_def);
base_service_def->name = clone_sharename;
smbconf_create_set_share(conf_ctx, clone_sharename, base_service_def);