Declared prototype of functions in sh1.c

invoke_cmd, read_cmd, parse_cmd, free_cmd, xmalloc, xrealloc
Implement a main function in sh1.c
Implement a read_cmd function in sh1.c

This funcution read a string from stdio.
And it will store the string in variable named "buf".
When this string is NULL, sh1.c would finished.
But if not so, it returned parse_cmd(buf).