FUSE is needed for wendelin.core, and if we don't check that FUSE works
inside and activate user namespaces on e.g. Linux 4.9 kernel,
wendelin.core will fail to function. Since today wendelin.core is
included into ERP5 as its base component(*) it is practical to require
FUSE-in-userns support unconditionally.

FUSE in user-namespaces started to work in Linux 4.18(+). Detect if it
should work inside via checking whether running kernel is newer. I
choosed this way for simplicity not to unroll test filesystem to try to
mount inside and also not to slow-down startup time.

(*) see slapos!874
(+) see https://git.kernel.org/linus/da315f6e0398 and https://git.kernel.org/linus/8cb08329b080.

/reviewed-by @jerome
/reviewed-on !13