mem: add internal API to get and set segment fd

mem: add internal API to get and set segment fd

Enable setting and retrieving segment fd's internally.

For now, retrieving fd's will not be used anywhere until we
get an external API, but it will be useful for things like
virtio, where we wish to share segment fd's.

Setting segment fd's will not be available as a public API
at this time, but internally it is needed for legacy mode,
because we're not allocating our hugepages in memalloc in
legacy mode case, and we still need to store the fd.

Another user of get segment fd API is memseg info dump, to
show which pages use which fd's.

Not supported on FreeBSD.

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>