Dear Fortranners,

Fortran 2018 added a QUIET= specifier to STOP and ERROR STOP statements.
Janne already implemented the library side code four (4!) years ago,
but so far the frontend implementation was missing.

Furthermore, F2018 allows for non-default-integer stopcode expressions
(finally!).

The attached patch provides this implementation.

That was not too much fun for the following reasons:

- fixed format vs. free format
- F95 and F2003 apparently did not require a blank between STOP and
  stopcode, while F2008+ do require it.

This should explain for the three testcases.

Regtested on x86_64-pc-linux-gnu.  OK for mainline?

One step closer to F2018!

Thanks,
Harald