sysctl snippet: move to /usr/lib and prefix with 10-

following best-practices according to `sysctl.d(5)`:
* Packages should install their configuration files in /usr/lib/ ...
* It is recommended to prefix all filenames with a two-digit number
  and a dash ...

the conffile removal is inspired by how it was done in `procps` (one
of the few packages in the debian repository, which did this
transition) and by following `dpkg-maintscript-helper(1)` and
`deb-conffiles(5)` (the former recommending the latter)

The choice of 10- as prefix is due to pve-container shipping its
snippet with that prefix already. other packages use higher numbers
(e.g. systemd - 50-)

Tested on 2 VMs (one with modifications, the other without) - worked
as advertised (the modified file was kept as
/etc/sysctl.d/pve.conf.dpkg-old and the upgrade notified me of the
change)

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>

debian/conffiles

@@ -0,0 +1 @@
+remove-on-upgrade /etc/sysctl.d/pve.conf

debian/pve-cluster.install

@@ -1,4 +1,4 @@
-debian/sysctl.d/pve.conf etc/sysctl.d
+debian/sysctl.d/10-pve.conf usr/lib/sysctl.d
 usr/bin/create_pmxcfs_db
 usr/bin/pmxcfs
 usr/lib/