Add MCL_ONFAULT to mlockall

This way, we don't fault in the entirety of our DSOs immediately;
instead, used pages are "sticky" in memory. Works only on kernel 4.4
and up: downlevel, we ignore the mlockall failure.

Once we get statically-linked lmkd in better shape, we'll just switch
to that.

Change-Id: I07a75ee3bc1264a1db41635c2acf611fede99b91