Implement wait() and waitpid() for Child Process Reaping

Implement wait() and waitpid() system calls for process reaping.

Requirements:

• Implement wait(&status) that blocks until any child terminates
• Implement waitpid(pid, &status, options) for specific child
• Handle WNOHANG option: return 0 immediately if no child ready
• Handle WUNTRACED option: also return for stopped children
• Encode exit status in integer (exit code in upper 8 bits, signal in lower 8 bits)
• Provide macros: WIFEXITED, WEXITSTATUS, WIFSIGNALED, WTERMSIG
• Remove zombie from process table after reaping
• Return -1 and set errno if no children exist
• Demonstrate zombie creation and reaping
• Show orphaned children being adopted

Test scenarios:

• Parent waits for single child
• Parent waits for multiple children (waits for first to exit)
• waitpid waits for specific child
• WNOHANG returns immediately if no child exited
• Zombie accumulation without wait()
• Orphan adoption when parent dies