Files which are installed as-is (any .service and other unit files, .conf
files, .policy files, etc), are left as is. My assumption is that SPDX
identifiers are not yet that well known, so it's better to retain the
extended header to avoid any doubt.
I also kept any copyright lines. We can probably remove them, but it'd nice to
obtain explicit acks from all involved authors before doing that.
If the new item is inserted before the first item in the list, then the
head must be updated as well.
Add a test to the list unit test to check for this.
Similar in function to LIST_INSERT_AFTER, this will insert a new element
into the list before the specified position. If the specified position
is NULL, the element is added as the tail of the list.
