Allow host to provide individual guests with terminal write access

[lostintangent]

When a guest attempts to write to a read-only terminal, it allows them to dynamically request write permission from the host. However, the host can only accept this request by providing write access to all guests in the session, which may not be desirable.

To improve upon the host's security controls, we should allow the host to provide terminal write access to only specific guests (as opposed to everyone). This would reflect the behavior we already support for file permissions, since the host can share in read-only mode, and then grant write access to individual guests.

[lostintangent]

Closing this since we haven't heard any customer requests for it. We can always re-open it if/when we do.