Merge sort from Prof Shene's book

One of a less well-known book by Prof Ching-Kuang Shene is 名題精選百則：技巧篇.  It is presented in C (my favorite) and most of the code are neat.

For example, I found the merge sort in the book is probably the clearest for a merge sort requires an extra memory.   That's probably become the merge sort I will write in future.