Add linked lists implementation.

As discussed on the mailing lists, the various linked list
implementations we currently use at a dozen of different places in the
calcurse source tree are inconvenient and should be replaced by a single
generic solution.

This is a first approach to introduce such a generic implemetation. It
provides following functions:

* llist_init(): Initialize a list.

* llist_free_inner(): Loop through a list and free all items.

* llist_free(): Free the list itself (but not the individual items).

* llist_first(): Get the first item of a list.

* llist_nth(): Get the nth item of a list.

* llist_next(): Get the successor of a list item.

* llist_find_first(): Find an item using a callback function.

* llist_find_next(): Find the next match using a callback function.

* llist_find_nth(): Find the nth item in a list (using a callback).

* llist_get_data(): Get a pointer to the actual data of a list item.

* llist_add(): Add an item at the end of a list.

* llist_add_sorted(): Add an item to a sorted list (using a comparison
                      callback function).

* llist_remove(): Remove an item from a list.

Linked lists are stored in "llist_t" structures, list items are to be
stored in "llist_item_t" structs.

All of the llist_*() functions either expect a pointer to a llist_t
structure (in case the function operates on the list itself) or a
pointer to a llist_item_t (llist_*_next() and llist_get_data()).

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/calcurse.h

@@ -54,6 +54,7 @@
 #include <stdlib.h>
 #include <stdio.h>
 
+#include "llist.h"
 #include "htable.h"
 
 /* Internationalization. */