Remove warnings from supergroups code on x86

Remove warnings from supergroups code on x86

Prototype functions where necessary, remove unused variables, fix
mismatches of data types (char * != void *), initialize variables the
compiler can't tell are initialized, compare integers against 0 and
not NULL, and wrap assignments used as conditions in an explicit
comparison.

This removes all warnings that fail --enable-checking on x86 with
--enable-supergroups.  64-bit will still have warnings.

Change-Id: I5ab6e317a280bea29cb44b97f2805a00e387577f
Reviewed-on: http://gerrit.openafs.org/1050
Tested-by: Russ Allbery <rra@stanford.edu>
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Reviewed-by: Derrick Brashear <shadow@dementia.org>