Derives one atomic lock from two nonatomic lockfiles with temporary content.

Suitable for use across containers.

Be sure to use a filename that is visible across all relevant containers.
If you run: fsock = fslock_mod.Fslock('/mnt/lockfile')
...then it will use /mnt/lockfile.1 and /mnt/lockfile.2

Tested and works on CPython 3.3-3.7, CPython 2.7, pypy2 7.1.1 and pypy3 7.0.0. Does not work on Micropython 1.11.

Threading support attempted, but not tested at all.
-----
It's remarkable how well this does, but I'm not 100% sure it's atomic.  I've tested it a lot, but without
Fslock.release(force=True), it gets confused under load - that is, just Fslock.release() fails.  Somehow being Less
cautious works fine.