mem: support --in-memory mode

mem: support --in-memory mode

Implement the final piece of the in-memory mode puzzle - enable running
DPDK entirely in memory, without creating any files.

To do it, use mmap with MAP_HUGETLB and size flags to enable DPDK to work
without hugetlbfs mountpoints. In order to enable this, a few things needed
to be changed.

First of all, we need to allow empty hugetlbfs mountpoints in
hugepage_info, and handle them correctly (by not trying to create any
files and lock any directories).

Next, we need to reorder the mapping sequence, because the page is not
really allocated until the page fault, and we cannot get its IOVA
address before we trigger the page fault.

Finally, decide at compile time whether we are going to be supporting
anonymous hugepages or not, because we cannot check for it at runtime.

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>