Implement the basics of dupoffs in ch05
Add checks for read and write in dupoffs in ch05

Implement checks for moving the offset by calling read and write on file
descriptors and run mulitple checks on different file descriptors.
Create README.md and Makefile for 5-3
Read and print command line options in 5-3
Implement writing file in 5-3

* Implement writing of files in 5-3 with or without append.
* Create script runtest.sh for running the tests.