[gfortran, testsuite] Fix bounds checking for substrings, enable bounds chekcing in dg testsuite, assorted fixes

[FX Coudert]

> 2. it fixes bounds checking for substrings.  Specifically,  
> something like "a"(i+1:i) is perfectly valid, but gave a runtime  
> error.

This front-end part of the patch is OK.

FX