Remove warnings related to type-punning

Remove warnings related to type-punning

cmd/cmd.c:
 - Just make dummy be a struct cmd_item instead of casting

rxdebug/rxdebug.c:
 - Access the stats packet through a union instead of casting and
   dereferencing

util/uuid.c:
 - Access the seed through a union and an array instead of that
   incrementing-pointer approach

Reviewed-on: http://gerrit.openafs.org/599
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>