The appropriate entry in /etc/group as returned by getgrnam() is
used to determine if 'su root' may be permitted, rather than
checking if membership exists in the result of getgroups().
The following changes were made regarding the behaviour of the special
group for 'su root'
* allow for definition of SUGROUP (defaults to "wheel") to override group name.
* use getgrnam(SUGROUP) instead of getgrgid(0).
* only scan getgrnam(SUGROUP)->gr_mem when checking for group membership.
* be more specific as to why 'su root' failed
NOTE: If a user's primary group is SUGROUP, and they're not a member
of SUGROUP in /etc/group, they will not be able to su.
_PASSWORD_WARNDAYS from <pwd.h>). For non-root users, enforce expiry when
it happens. From Simon Gerraty <sjg@zen.void.oz.au> in [bin/935].
* Check for group 0 in process's current group membership (as returned by
getgroups(2)), instead of just looking at the entry for wheel in /etc/group.
Based on code by Dan Caresone <dan@oink.geek.com.au> in [bin/792], and
also solves [bin/2466].
* Clean up to pass -Wall
