linux container usage: collect from host's /proc volume mounted in /host_proc

This feature allows collection of metrics of the host machine from a linux container running `collectd`.
Hardcoded path of `proc` in the source code is replaced with `host_proc`.

To use this feature, mount the host directory `/proc` as a data volume `/host_proc` in the linux container.
For example, this can be done in docker using the command `docker run -v /proc:/host_proc <docker-image-name>`.