hurd: Use mach_msg_type_number_t where appropriate

It has been decided that on x86_64, mach_msg_type_number_t stays 32-bit.
Therefore, it's not possible to use mach_msg_type_number_t
interchangeably with size_t, in particular this breaks when a pointer to
a variable is passed to a MIG routine.

Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>
Message-Id: <20230212111044.610942-3-bugaevc@gmail.com>