Add option to hide processes running in a container

[valdaarhun]

Might close #1036

The patch makes use of NSpid to check whether a process runs inside a pid namespace that is different from the host's init process' pid namespace. If this check is true, the cgroup name is checked to ensure it isn't a process running on the host that happens to run in a different pid namespace.

[BenBE]

Please have a look at the compile issues mentioned by the CI. There seem to be some headers missing for iswdigit AFAICS.

[valdaarhun]

Feel free to clean up the history on your PR as much as you like

Cool. I'll clean up the commit history and push it again.

[valdaarhun]

I have cleaned up the commit history as best as I could.

[m0vie]

The O keyboard shortcut is not documented in the F1 help. Could this also be added? Same for the man page.

[valdaarhun]

@m0vie Hi, thanks for pointing this out. This must have slipped my mind. It's been resolved in #1250 .