conf: implement resource limits

conf: implement resource limits

This adds lxc.limit.<name> options consisting of one or two
colon separated numerical values (soft and optional hard
limit). If only one number is specified it'll be used for
both soft and hard limit. Additionally the word 'unlimited'
can be used instead of numbers.

Eg.
  lxc.limit.nofile = 30000:32768
  lxc.limit.stack = unlimited

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>