vzctl: add ability to skip creation of veth devices (v2)

It will be used for resuming CT with help CRIU.
CRIU restores veth devices and configures them inside CT,
so vzctl should configures them on the host side.
cpt: restore veth devices with correct names (v4)

transmit pair of veth names to criu via the option --veth-pair

v2: unset IFS and delete eval from vps-rst
v3: fix comments from Kir
v4: vzcheckvar VE_VETH_DEVS
[RFC] cpt: clean up dump files

OpenVZ checkpointing saves all data in one file.
Currently criu is used for upstream CTs and it generates many files (one
file for each type of objects), so we can get the following error

Stale CT dump file /var/lib/vz/dump/Dump.101 found, removing
Can't unlink /var/lib/vz/dump/Dump.101

Only a tools, which checkpoints CT, knows how to delete own data,
so lets do that in the script vzt-rst.
sync
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