mmu: mark address parameter as const

shmdt() is defined to get const pointer. Next patch implements shmdt
using unmap(), so const qualifier either need to be casted off or
unmap() and friends need to get const pointer as a parameter too. This
patch implement the later.

Signed-off-by: Gleb Natapov <gleb@cloudius-systems.com>
libc: implement System V shared memory interface

Implement shmget/shmat/shmdt/shmctl function required by System V shared
memory interface.

Signed-off-by: Gleb Natapov <gleb@cloudius-systems.com>
tests: add System V shared memory test

Signed-off-by: Gleb Natapov <gleb@cloudius-systems.com>