You can now open as many PTY pairs as you like. Well, it's actually capped
at 8 for now, but it's just a constant and trivial to change.
Unregistering a PTY pair is untested because I didn't want to start
mucking with that in Terminal right now.
When you open /dev/ptmx, you get a file descriptor pointing to one of the
available MasterPTY's. If none are available, you get an EBUSY.
This makes it possible to open multiple (up to 4) Terminals. :^)
To support this, I also added a CharacterDevice::open() that gets control
when VFS is opening a CharacterDevice. This is useful when we want to return
a custom FileDescriptor like we do here.
