backup: restore: keep directories in /etc/pmg for inotify

backup: restore: keep directories in /etc/pmg for inotify

By wiping the subdirectories in /etc/pmg/, we lose the inotify
watchers upon restore (/etc/pmg itself and thus most configs are
currently handled by the keep_root flag to rmtree)
This can lead to inconsistencies after restoring for parts relying on
config in a subdirectory (e.g. /etc/pmg/pbs/pbs.conf).

This patch uses File::Find (included in perl-modules-$perlver) to keep
all directories an unlink everything else.
This was chosen for future robustness over keeping an explicit list of
directories to keep, in case a new directory gets added.

quickly tested with a fifo, chardev, and socket in the directory.

an alternative approach would be to simply reload pmgdaemon/pmgproxy
upon config-restore, but that feels more likely to miss some
(potentially future) service, expecting inotify to work.

Reported-by: Fiona Ebner <f.ebner@proxmox.com>
Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>
Tested-by: Fiona Ebner <f.ebner@proxmox.com>