New functions GetFrom{Pid,Thread}WithAltProcfs

[alban]

Existing functions GetFrom{Pid,Thread}() need the pid/tid argument to be numbered as per the pid namespace of the /proc mount.

In some container configurations, this is not suitable: a container with unshared pid namespace and the host's /proc mounted in /host/proc needs to lookup pids numbered as per the host pid namespace in /host/proc.

GetFrom{Pid,Thread}WithAltProcfs() provide a way for such container setups to use an alternative /proc path.