Eliminate some "implicit function declaration" warnings

Eliminate some "implicit function declaration" warnings

Include appropriate header files to eliminate some warnings for
implicit function declarations.  These are simple cases that don't
generate new warnings because of the prototypes.
Use grp.h if it exists to get setgroups() definition.

Reviewed-on: http://gerrit.openafs.org/233
Tested-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Tested-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>