core/unit: append newline when writing drop ins

core/unit: append newline when writing drop ins

unit_write_drop_in{,_private}{,_format} are all affected.

We already append a header to the file (and section markers), so those functions
can only be used to write a whole file at once. Including the newline at
the end feels natural.

After this commit newlines will be duplicated. They will be removed in
subsequent commit.

Also, rewrap the "autogenerated" header to fit within 80 columns.