ENH: support true/false, yes/no values for FOAM_ABORT (#1896)

- consistent with FOAM_SIGFPE etc.

- centralize code as error::useAbort() static function to avoid
  scattering the logic throughout the code.

ENH: also accept "0" and "1" string values for Switch

- not the normal path for Switch input (eg, from a dictionary), but
  consistent with bool definitions and simplifies string parsing.

  This means that `FOAM_SIGFPE=1 application` will now also work.

src/OpenFOAM/db/functionObjects/functionObjectList/functionObjectList.C

@@ -71,7 +71,7 @@ namespace Foam
     //- Mimic exit handling of the error class
     static void exitNow(const error& err)
     {
-        if (hasEnv("FOAM_ABORT"))
+        if (error::useAbort())
         {
             Perr<< nl << err << nl
                 << "\nFOAM aborting (FOAM_ABORT set)\n" << endl;