perform range checking in accordance to GDB rules

The release423 branch supports OCCURS and reference-modification quite well. It does implement some range checks, too, these should be adjusted to work depending on GDB's Range Checking (as with modula-2: default to output an error and ignore the expression, but be adjustable to "warning" and "ignore".

Background: early COBOL had range errors declared as "undefined", in COBOL2002 exception handling was added (which is done in GnuCOBOL depending on the -fec flags, and --debug implies -fec=all). Programs that were not compiled with those flags on work "in general" identically as in MF or on IBM: they calculate the data offset (often pointing to a complete other variable, possibly also into program space which will then raise a SIGSEGV on write). While the results differ between compilers if the 01-level is left 1 COBOL and so GnuCOBOL does allow that in principle and it would be good to also allow this controlled in the debugger.

  1. different compilers have all the data "in order of definition" and with different alignment in memory, GnuCOBOL has - on most machines an backwards order, aligned on 8 bytes (GC4 has dialect options to adjust that to match all known compilers) ↩