pidns: virtualize pid_max

This feature makes kernel.pid_max sysctl be per pid namespace (instead of a global sysctl by default).

Q: Why do we need this feature at all? A:

  1. historically is was implemented to support 32bit containers (pids with the value greater than 2^32 make 32bit userspace go mad)

  2. this feature is needed for Containers online migration: if someone tries to migrate a process with pid XXX to the Hardware Node with pid_max < XXX, the migration will fail without iper-pidns pid_max support

https://jira.sw.ru/browse/PSBM-140308