--enable-languages should be "fortran" instead of "f95"

[François-Xavier Coudert]

There are very logical/meaningful arguments to renaming the language
name from f95 to fortran (this is PR 17815). Is there any technical
reason why we shouldn't do it now? At least, we could still support
the old name for some time (at least on 4.0 branch), but is there any
reason for delaying (possibly for an infinite time) such a change?

If there is no argument against it, I will provide the patch needed. Comments?

FX