Source parsing: consider end program with no program header

[perazz]

Address #1075 @ivan-pi.

This PR improves parsing of Fortran programs with no program header.

Note: some very special cases are still not being considered, due to the limitations in the current parser.

For example, the following valid program unit cannot be properly parsed yet, because there is currently no mechanism to flag whether we're currently inside a subroutine or function tree:

subroutine a(i)
   integer :: i
   call other(i+2)
   contains
   subroutine other(j)
      integer :: j
      print *, j
   end 
end
call a(123)
end 

[perazz]

Let's wait a bit longer, and then merge if there are no further comments.