Consolidate repeated macro definitions into one header

The MIN, MAX, CLAMP, MINIMUM, and MAXIMUM macros appear
throughout the codebase with many re-definitions.  Make
a single copy of each in a common header file, and use
the BSD variants of MINIMUM/MAXIMUM due to conflicts in
the system <sys/param.h> headers.

darwin/Platform.c

@@ -22,10 +22,6 @@ in the source distribution for its full text.
 #include <stdlib.h>
 
 
-#ifndef CLAMP
-#define CLAMP(x,low,high) (((x)>(high))?(high):(((x)<(low))?(low):(x)))
-#endif
-
 ProcessField Platform_defaultFields[] = { PID, USER, PRIORITY, NICE, M_SIZE, M_RESIDENT, STATE, PERCENT_CPU, PERCENT_MEM, TIME, COMM, 0 };
 
 const SignalItem Platform_signals[] = {

darwin/Platform.h

@@ -14,10 +14,6 @@ in the source distribution for its full text.
 #include "BatteryMeter.h"
 #include "DarwinProcess.h"
 
-#ifndef CLAMP
-#define CLAMP(x,low,high) (((x)>(high))?(high):(((x)<(low))?(low):(x)))
-#endif
-
 extern ProcessField Platform_defaultFields[];
 
 extern const SignalItem Platform_signals[];