sys: lists: Add list functions to compute list length

This commit adds functions to slist, sflist, and dlist to return the
length of provided lists, which will allow future commits to refactor
code that implements this manually.

The new functions all take a reference to any node in the list and compute
the length of the entire list. If the list is empty, they return 0.

Signed-off-by: J M <montytyper@msn.com>