Update hurd/hurdselect.c to be more portable.

Summary of changes:
- Use BAD_TYPECHECK to perform type checking in a cleaner way.
  BAD_TYPECHECK is moved into sysdeps/mach/rpc.h to avoid duplication.
- Remove assertions for mach_msg_type_t since those won't work for
  x86_64.
- Update message structs to use mach_msg_type_t directly.
- Use designated initializers.
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