[STYLE] Make IO redirect names more clear

[itislu]

• Rename handle_io_redirect() to open_io_files().
• Rename stdio_bind.c to stdio_redirect.c because "binding" file descriptors is commonly used with sockets, not with files.

[LeaYeh]

I still think handle_* is more make sense for me

[itislu]

I still think handle_* is more make sense for me

The problem is that f.e. handle_red_out() sounds like it will handle the redirection completely, meaning it will redirect STDOUT to a file. But that's not what that function does. The actual redirection with dup2() is done by the redirect_*() functions.
I never know what the handle_*() functions actually do.