Added a vzdump.conf option to controll gzip compression.
When 'pigz' (defaults to 0) is >0, pigz support is enabled.
When the pigz option equals 1 pigz uses #cores/2 threads,
else it spawns N threads. To use it select gzip in the web
interface and set the aproppriate option in /etc/vzdump.conf
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>