Add pressure stall information (PSI) meters on Linux

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.

linux/Platform.h

@@ -45,4 +45,6 @@ void Platform_setSwapValues(Meter* this);
 
 char* Platform_getProcessEnv(pid_t pid);
 
+void Platform_getPressureStall(const char *file, bool some, double* ten, double* sixty, double* threehundred);
+
 #endif