pmg7to8: Add check for dkms modules

pmg7to8: Add check for dkms modules

... and warn if at least one is present and PMG is running not running
in a container.

follows commit 0329876ccf1d78b848897718bb0c2337c6a55fbb in pve-manager

with the change that the dkms check is skipped if
proxmox-mailgateway-container is the installed meta-package (as we do
for the kernel checks).

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