Add a way to let other gems handle closing of fds in mruby-io

[Asmod4n]

I'm currently writing a gem which wants to integrate with mruby-io and mruby-socket.
At the moment mruby-io closes fds it didn't create itself and thus can cause problems when other gems except the fd to still be available.

This only changes how fptr_finalize handles the closing of fds and nothing else.
I've tried handling that in my gem by stopping a created mruby-io object from being garbage collected until it's not needed anymore, but that wasn't enough.

[Asmod4n]

A short explanation: i'm writing a mruby gem to implement io_uring from linux.
This is fixing a otherwise impossible to handle race condition.

I've tried several ways to fix this in my gem, the only thing that worked is to mrb_gc_register every mruby-io object i created and never unregister them, because you can't know when its save to release them.