Fix incorrect freemem report in docker containers

[durigon]

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When using the Erlang agent to monitor OS, I found that the free memory collection in the docker container was incorrect.

We use 'MemAvailable' to get a free memory in the linux system when using the erlang agent.

MemAvailable %lu (since Linux 3.14) An estimate of how much memory is available for starting new applications, without swapping.

In a container, MemAvailable should be the sum of MemFree, Cached and Buffers, but it is the same as MemFree. After all, we can be mistaken for a memory leak because it does not include Cached/Buffers memory.

docker-container# egrep "MemFree|MemAvail|^Cached|Buffers" /proc/meminfo
MemFree: 93009720 kB
MemAvailable: 93009720 kB
Buffers: 0 kB
Cached: 7712588 kB

docker-container# uname -r
4.19.55

In a Host machine, MemAvailable is MemFree + Buffers + Cached.

host-machine# egrep "MemFree|MemAvail|^Cached|Buffers" /proc/meminfo
MemFree: 339513028 kB
MemAvailable: 381096504 kB
Buffers: 15072 kB
Cached: 37029112 kB

host-machine# uname -r
4.19.55

[How to improve] Use the sum of MemFree, Cached, and Buffers as real available memory instead of MemAvailable to improve the absence of MemAvailable field in old linux kernel version(prior to 3.14) or to improve the incorrect MemAvailable in docker containers.

This PR solves issue #376