updatecerts: create base directories of observed files

updatecerts: create base directories of observed files

replaces the random hacks where we do some hail-mary mkdir in a
writer or the like, to ensure that the directory structure exists and
we can write safely.

more central and safer would be pmxcfs itself, but to late in the
release cycle to do that now.

Chicken out if pmxcfs is not mounted, we don't want to trash it's
(future) mountpoint..

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>