Make the semantics if VG_(getgroups) match those of getgroups(2).
That is, add the special case (size == 0) to query the number of
supplementary group IDs. This simplifies the call site nicely.


git-svn-id: svn://svn.valgrind.org/valgrind/branches/BUF_REMOVAL@14593 a5019735-40e9-0310-863c-91ae7b9d1cf9