drivers/pty: Destroy TTYs when usage is complete.

Otherwise, they will be "reused" and persist settings
and buffers, which we don't want.

Fixes the remaining part of #18488.

drivers/pty: Close "slave" TTYs when the "master" is closed.

The old TTY driver did this, but it was not properly carried
over into this new driver.

Fixes the first part of #18488: the child bash process from the
first session is properly closed, but the TTY state isn't reset
and there can still be stale things left in its buffer, it appears.

tty: Implement exclusive mode

Implemented exclusive mode on Haiku and added the related `ioctl`
operations (`TIOCEXCL` and `TIOCNXCL`).

Change-Id: Iaa201ea20eec0e45d02dd5db9ba6aa35fd27dfb2
Reviewed-on: https://review.haiku-os.org/c/haiku/+/6387
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: Jessica Hamilton <jessica.l.hamilton@gmail.com>

drivers/pty: Check slave TTYs also in get_tty_index.

The old TTY driver had one set of indexes for both master
and slave TTYs, and it used those indexes interchangeably.
We thus need to check both lists here also.

Fixes SSH passphrase prompts not working.

Replace the "tty" driver with a "pty" driver.

This new driver uses the "generic" TTY layer, unlike the old driver
which had its own implementation (which the generic module was derived
from, originally.)

The remaining bits of support for controlling TTYs is added to the kernel &
generic layer at the same time, which should allow for serial interfaces
to be controlling terminals now, as well.

Tested with bash, nano, vim; all seems to still be working as expected.