Sometimes, we want to use CList to track a simple data item. But contrary
to GList/GSList, we need to define a structure to hold the data pointer
and the CList member.
Add a generic NMCListElem type that can be used for such simple uses.
Before you ask: why not use GList/GSList? Because even simple operations
like g_list_append() is O(n), which kinda defeats the purpose of having
a doubly linked list.
This code is added to a new header file nm-c-list.h, the reason is that
there is no other good place:
- "nm-utils/c-list.h" is a clone of upstream, it should not deviate.
- "nm-utils/c-list-util.h" contains our utils functions for c-list.h
but should be plain C, independent of glib.
- "nm-utils/nm-shared-utils.h" contains our glib related utilities,
but it should not drag in "c-list.h".
So, "nm-c-list.h" is a utility libray that extends "c-list.h" and
requires glib.
