fix #4192: revamp check for systemd version

fix #4192: revamp check for systemd version

Instead of iterating through several folders, it might just be easier to
check the objdump output of /sbin/init and getting the version from there.
Resolving the /sbin/init symlink happens inside the chroot, but the
objdump from the host system is used, as to not run any untrusted
executables.

Signed-off-by: Leo Nunner <l.nunner@proxmox.com>