[teleport-update] Add unlink-package command

[sclevine]

This PR adds the unlink-package subcommand to the teleport-update command.

This subcommand reverses the linking created in #48712, to support removal of deb/rpm packages.

The link-package subcommand will allow the teleport RPM and DEB packages to co-exist with versions of Teleport that are installed by the teleport-update binary, by ensuring that Teleport binaries installed by the package into /usr/local/teleport-system are always linked to /usr/local when no teleport-update-managed installation is present.

Note that the Teleport package will have its files moved into /usr/local/teleport-system in a future PR. This command is not useful until this packaging change is complete.

The teleport-update binary will be used to enable, disable, and trigger automatic Teleport agent updates. The new auto-updates system manages a local installation of the cluster-specified version of Teleport stored in /var/lib/teleport/versions.

RFD: #47126
Goal (internal): https://github.com/gravitational/cloud/issues/10289

Examples:

ubuntu@legendary-mite:~$ sudo ./teleport-update link-package
2024-11-20T06:40:30Z INFO [UPDATER]   Systemd configuration synced. unit:teleport.service agent/process.go:255
2024-11-20T06:40:30Z INFO [UPDATER]   Successfully linked system package installation. agent/updater.go:718
ubuntu@legendary-mite:~$ ls -la /usr/local/bin/
total 8
drwxr-xr-x  2 root root 4096 Nov 20 06:39 .
drwxr-xr-x 11 root root 4096 Nov 13 01:41 ..
lrwxrwxrwx  1 root root   46 Nov 20 06:39 fdpass-teleport -> /usr/local/teleport-system/bin/fdpass-teleport
lrwxrwxrwx  1 root root   35 Nov 20 06:39 tbot -> /usr/local/teleport-system/bin/tbot
lrwxrwxrwx  1 root root   35 Nov 20 06:39 tctl -> /usr/local/teleport-system/bin/tctl
lrwxrwxrwx  1 root root   39 Nov 20 06:39 teleport -> /usr/local/teleport-system/bin/teleport
lrwxrwxrwx  1 root root   34 Nov 20 06:39 tsh -> /usr/local/teleport-system/bin/tsh
ubuntu@legendary-mite:~$ sudo ./teleport-update unlink-package
2024-11-20T06:41:28Z INFO [UPDATER]   Systemd configuration synced. unit:teleport.service agent/process.go:255
2024-11-20T06:41:28Z INFO [UPDATER]   Successfully unlinked system package installation. agent/updater.go:732
ubuntu@legendary-mite:~$ cat /lib/systemd/system/teleport.service 
cat: /lib/systemd/system/teleport.service: No such file or directory
ubuntu@legendary-mite:~$ ls -la /usr/local/bin/
total 8
drwxr-xr-x  2 root root 4096 Nov 20 06:41 .
drwxr-xr-x 11 root root 4096 Nov 13 01:41 ..