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I was occasionally passing negative values to size_t.
Plus, this better reflects the intent of the variant argument.

Reported by Coverity:
https://scan8.coverity.com/reports.htm#v13253/p10402/fileInstanceId=22093891&defectInstanceId=7543346&mergedDefectId=174179&fileStart=251&fileEnd=500

Calls marked with xSnprintf shouldn't fail.
Abort program cleanly if any of them does.

Specifically, Platform_signals[] and Platform_numberOfSignals. Both are
not supposed to be mutable. Marking them 'const' puts them into rodata
sections in binary. And for Platform_numberOfSignals, this aids
optimization (aids only Link Time Optimization for now). :)
Signed-off-by: Kang-Che Sung <explorer09@gmail.com>

With the CLAMP macro replacing the combination of MIN and MAX, we will
have at least two advantages:
1. It's more obvious semantically.
2. There are no more mixes of confusing uses like MIN(MAX(a,b),c) and
   MAX(MIN(a,b),c) and MIN(a,MAX(b,c)) appearing everywhere. We unify
   the 'clamping' with a single macro.
Note that the behavior of this CLAMP macro is different from
the combination `MAX(low,MIN(x,high))`.
* This CLAMP macro expands to two comparisons instead of three from
  MAX and MIN combination. In theory, this makes the code slightly
  smaller, in case that (low) or (high) or both are computed at
  runtime, so that compilers cannot optimize them. (The third
  comparison will matter if (low)>(high); see below.)
* CLAMP has a side effect, that if (low)>(high) it will produce weird
  results. Unlike MIN & MAX which will force either (low) or (high) to
  win. No assertion of ((low)<=(high)) is done in this macro, for now.

This CLAMP macro is implemented like described in glib
<http://developer.gnome.org/glib/stable/glib-Standard-Macros.html>
and does not handle weird uses like CLAMP(a++, low++, high--) .

- currently implemented for darwin and linux

FreeBSD
Linux
Other platforms will have it undefined for now.

Implementations for Linux (tested) and FreeBSD (still untested, thanks to @etosan for providing the table).
Darwin and OpenBSD(ping @mmcco) builds should be broken now, pending their own tables.

gcc gives warnings like this:

warning: ignoring return value of ‘fscanf’, declared with attribute
warn_unused_result

Assign value to a variable, cast to (void) to discard it.

Conflicts:
	Process.c
	Process.h
	ProcessList.c
	ScreenManager.c
	linux/LinuxProcessList.c

Kinda runs, but functionality from the original main loop
is still missing. Patience.

This is a work-in-progress, code is currently broken.
(Some actions, and notably, the header, are missing.)

Set up environment to move other meters.