Fix various ftw/nftw "shall fail"s from POSIX.

POSIX says ftw/nftw "shall fail" in various cases where BSD's fts_open
doesn't. Since our ftw/nftw are written in terms of fts_open, add a back
door so we can hint to ourselves when we should have the POSIX semantics.

Also pull several O_CLOEXEC and don't-null-check-before-free cleanups
from upstream, and add a couple of tests.

Bug: http://b/31152735
Test: ran bionic tests and LTP "nftw01" test
Change-Id: Ib05facacc1da4c8b2ab48e9ecce88f11a5406630