The setting of the group vector, the supplemental group membership, of a process
for any target user other than root, is currently hard-coded.  I can see why
preserving the current group vector when the target user is root is beneficial,
but user root should not be treated as a hard-coded exception -- this behavior
should be controllable for any target user, via the command-line.

I've attached a patch which adds a -P command-line option, with appropriate
changes to the sudo man page.