Update f90-insert-namelist-safe to use f90-next-namelist

This change also means we can take the namelist name as an input
 argument. Additionally, add an extra newline at the end of the namelist, to keep
 namelists on separate lines, now that we can add namelists from inside other
 ones; change (previous-line) to (forward-line -2) to place point at the start of
 the line inside the new namelist. This last change is in anticipation of a
 separate function to handle indentation.