Save domain and GID for groups from the configured domain
Currently users from subdomains can only be members of groups from the
configured domain and to access those groups a pointer to the domain
struct of the configured domain is used. This patch sets the dom_grp
member of struct pac_grp to point to the domain struct of the configured
for groups from this domain. This is a first step to allow group
membership for groups from subdomains as well. For those groups a
pointer to the related subdomain structure will be saved.