mmu: VMA permission flags

Add permission flags to VMAs. They will be used by mprotect() and the
page fault handler.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
mmu: Anonymous memory demand paging

Switch to demand paging for anonymous virtual memory.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
tst-mmap.so: mincore() tests for demand paging

As suggested by Nadav, add tests for mincore() interraction with demand
paging.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>