Before this patch, after `xslapos node stop wcfs`, `xslapos node start wcfs`
was failing in a sense that started service was turning into EXITED state
with something like the following in its log:

    wcfs: 2021/05/31 15:28:15 zodb: FIXME: open zeo://10.0.82.110:2100?storage=root: raw cache is not ready for invalidations -> NoCache forced
    Traceback (most recent call last):
      File "/srv/slapgrid/slappart34/srv/runner/software/44fe7dd3f13ecd100894c6368a35c055/bin/wcfs", line 290, in <module>
        sys.exit(wendelin.wcfs.main())
      File "<decorator-gen-7>", line 2, in main
      File "/srv/slapgrid/slappart34/srv/runner/software/44fe7dd3f13ecd100894c6368a35c055/develop-eggs/pygolang-0.0.8-py2.7-linux-x86_64.egg/golang/__init__.py", line 103, in _
        return f(*argv, **kw)
      File "/srv/slapgrid/slappart34/srv/runner/software/44fe7dd3f13ecd100894c6368a35c055/parts/wendelin.core/wcfs/__init__.py", line 409, in main
        serve(zurl, optv, exec_=True)
      File "/srv/slapgrid/slappart34/srv/runner/software/44fe7dd3f13ecd100894c6368a35c055/parts/wendelin.core/wcfs/__init__.py", line 365, in serve
        f = open(mntpt + "/.wcfs/zurl")
    IOError: [Errno 107] Transport endpoint is not connected: '/dev/shm/wcfs/c63ef16eb2e74dc62cabf8ffc11798fd42d0158a/.wcfs/zurl'

That's because when wcfs is killed (xslapos node stop), it is only filesystem
server process that is terminated, but the FUSE mount and connection remains
registered on the system, and any access to FUSE filesystem with exited/crashed
server returns that "Transport endpoint is not connected" error.

-> Fix that by detecting at wcfs start whether it was previous unclean wcfs
shutdown, and then pre-clean things as necessary.