Use password/group cache functions and avoid stashing a pointer to

Use password/group cache functions and avoid stashing a pointer to
the return value of getgrgid(3) or getgrnam(3) which relies on
undefined behavior.  The rdist server will now use getgroups(2) to
determine group membership of the invoking user.  In addition, there
is now one implementation of tilde expansion instead of two.
OK tb@ tim@