Just need to check prev and next for NULL to know whether to mess
with the head and/or tail pointers.
Change-Id: I0aee057111e11735b7806e7214af0a6038f0ab53
ll_insert_next() and ll_remove_next() can be done more elegantly
by adding a level of indirection to reference the 'next' pointer.
Change-Id: If3ab2bc2a659b517c793749cfa9088938ae08d0d
Forms implemented to a greater or lesser degree at the moment:
ll_* = singly-linked list
lld_* = doubly-linked list
lldc_* = doubly-linked circular list
Change-Id: Ieed5af50fc59165c8b14c3513b3b5d0e6f7de9fa
