[gfortran] Scanner rewrite, fix for PR13702

[Tobias Schlüter]

Joseph S. Myers wrote:
> And (as Kenner keeps reiterating) functions should all have comments
> explaining what they do and what their parameters and return values mean.
> The context in this patch shows that it was a pre-existing condition that
> this function lacks such a comment.  Hopefully gfortran can get such
> comments while people still remember the intent from when the functions
> were written rather than trying to reverse engineer much later.
> 

Actually, people don't remember them for the scanner. Andy Vaught wrote 
it, and I think we all know what happened. Right now I understand the 
scanner very well, and I will add any missing comments after I return 
from a vacation that I'm leaving for tomorrow.

- Tobi