Create main.c

Commands cd, where, nice, setenv, unsetenv, logout, echo and pwd are builtins for ush. Pipes ( | , |&), I/O redirection( < , >, >> , >&, >>&) and pipelines are handled for all unix commands. Signals SigINT SigHUP and SigTERM are handled by ush. Also, initially, commands are read and executed from ~/.ushrc.