Initial Makefile

added a "prefix" buffer for more informative perror where applicable
added getcwd() to save the new path inside cd() if success
pwd() to print current_dir
echo(buf)
help, clr, pause placeholders

make_prompt(char*); helper function to format a shell prompt with %uid
($ or #) and %pwd

main()

extern FILE *stdin for initial prompt and command-line input