use Syscall module instead of raw syscall numbers

use Syscall module instead of raw syscall numbers

Raw syscall numbers were not platform independent, so replace them
with the new PVE::Syscall module which gets the syscall numbers from
syscall.ph, which provides the platform local values.

we cannot directly use syscall.ph as it is no normal perl module and
so it's usage requires special handling

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