Project 8

This project continues your exercise in the C programming language, to implement a 
linked list using a head and tail pointer.

1- Copy [repo]/class/proj8/* to [repo]/[username]/proj8. Best if you add an commit at this time.
2- Modify linkedlist-2.c to pass the test "make test".
3- Submit your project 8 by the due date.

The implementation of the list is left unfinished. You need to implement by writing code
in the four functions, list_create, list_push, list_add and list_cat.

* list_create: Use malloc to create an struct list and a struct node, and point the list
  head to the node. The list tail is set to NULL.
* list_push: As node_push, except update the list tail pointer. 
* list_add: As node_add, except update the list tail pointer.
* list_cat: Given two lists, concatenate the second list to the end of the first. This
  operation does not copy any struct nodes. The second list is no longer valid after
  the operation, so free the struct list and dummy struct node of the second list.
  
The test input gives basic functioning. More extensive tests might be applied to your
code for full credit.