Process: Fix AttributeError when stdin, stdout, and stderr are redirected to files

[jamestiotio]

This closes #2036.

This PR would allow users to redirect stdin, stdout, or/and stderr to files via 2 methods, even partially (i.e., the user can choose to redirect only stdin, only stdout, only stderr, or any combination of those 3):

1. By using Python's built-in open() function:

   pwn.process("cat", stdin=open("some_file", "r"), stdout=open("some_other_file", "w"), stderr=open("yet_another_file", "w"))
   

2. By using the os.open() function:

   pwn.process(["cat", "-"], stdin=os.open("some_file", os.O_RDWR), stdout=os.open("some_other_file", os.O_RDWR), stderr=os.open("yet_another_file", os.O_RDWR))
   

If the user passes in some raw integer(s) for stdin, stdout, or/and stderr, such as when they invoke os.open(), we only attempt to check whether they are valid file descriptors or not. Other than that, we trust the user that they know what they are doing.

[jamestiotio]

@Arusekk Could you kindly review this PR? Thank you!

[Arusekk]

It looks like this code is overly complex. I think we do not want to do any file descriptor validation, as they are foreign objects to Python code, and validating them is inherently racy.