Instead of trying to grow the buffer in make_grlist_item(), simply

Instead of trying to grow the buffer in make_grlist_item(), simply
increase the total length, free the old buffer and allocate a new
one.  This is less error prone and saves us from having to adjust
all the pointers in the buffer.  This code path is only taken when
there are groups longer than the length of the user field in struct
utmp or utmpx, which should be quite rare.