userspace code test: check the return status of the child

When the child exit normally, the WIFEXITED() will return true.
userspace code test: add breakpoint and resume the child

Adding breakpoint is very interesting.
1. retrive the instruction in child for a specific address.
2. set the first byte to 0xcc, which is the opcode of int3.
3. tell child to continue.
4. when child hit the breakpoint,
     restore the instruction to original one
     set EIP to original one, since at this time it points to addr+1

How to get the address for breakpoint.
1. use "objdump -d elf_file"