Deny Write on Executables

Deny write on executables.

Add code to deny writes to files in use as executables. Many OSes do this because of the unpredictable results if a process tried to run code that was in the midst of being changed on disk. This is especially important once virtual memory is implemented in project 3, but it can't hurt even now.

You can use file_deny_write() to prevent writes to an open file. Calling file_allow_write() on the file will re-enable them (unless the file is denied writes by another opener). Closing a file will also re-enable writes. Thus, to deny writes to a process's executable, you must keep it open as long as the process is still running.