Adds line wrapping to Fortran codegen call signatures.

Fortran has a maximum line length of 132 characters. The fcode printer handles
this for the expressions itself, but the call signature failed to wrap if the
number or length of arguments is too large. This fix should deal with most
normal cases.

Fixes issue #7934.
Added field numbers in .format call for Python 2.6 compatibility.
Merge pull request #7968 from moorepants/fortran-arg-line-wrap

Adds line wrapping to Fortran codegen call signatures.