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.