sftp: implement session timeout to automatically close idle connections

[rmetrich]

See https://bugzilla.mindrot.org/show_bug.cgi?id=3484

Currently there is no way for the sftp backend (sftp-server or internal-sftp) to close idle connections (by idle I mean no order sent for some time by the sftp client).

This is very problematic for SFTP servers because clients can remain connected, which consumes file descriptors and resources in general, causing potentially system limits to be reached.
This is a case I handled recently, where system-wide file descriptors were exhausted, due to left-opened sftp sessions + corresponding systemd sessions.

There are ClientAlive* properties but these only work for dead clients.

This PR implements the functionality through a new -t <timeout> option passed to sftp-server or internal-sftp executables.

[djmdjm]

I don't want to do this in sftp-server, see djmdjm/openssh-wip#16 for a more general mechanism

[djmdjm]

The more general mechanism was released in OpenSSH 9.2