Add pressure stall information (PSI) meters on Linux

[bluetech]

The pressure stall information (PSI) metrics provide useful information on delays caused by waiting for CPU, IO and memory. Particularly on busy servers it can provide a quick overview of what's "slowing things down".

This feature is supported on Linux >= 4.20. The interface is documented here: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/plain/Documentation/accounting/psi.txt These links provide rationale: https://lwn.net/Articles/759781/ https://facebookmicrosites.github.io/psi/

The following metrics are added, corresponding to the currently exposed lines (see head /proc/pressure/*):

• PressureStallCPUSome
• PressureStallIOSome
• PressureStallIOFull
• PressureStallMemorySome
• PressureStallMemoryFull

The color scheme is the same as that used for Load Average, however I gave it separate entries just in case someone wants to change them specifically.

Tested on 4.20.7-arch1-1-ARCH, on the linux platform. Also tested that other platforms still compile (changed configure to use the unsupported platform).

Closes #879.

[natoscott]

Merged here: htop-dev/htop@2be0992