Defines the macro HAVE_GETGROUPLIST if getgrouplist() is
available, and defines the type passed to the GID array as
GETGROUPS_T

On enterprise systems, using getgrouplist() (if available)
is more efficient than iterating over the members of the group,
and is also more likely to work

This check is now performed within g_check_user_in_group()

Improve the built-in access checks for sesman/sesexec:-
- Group existence is checked for at login-time rather than program
  start time
- The name of the group is now included in the message

Also, check for UID == 0 when checking for root, rather than just
checking the name (which might be an alias)