[teleport-update] Extended binary validations

[sclevine]

This PR adds additional validations on binaries downloaded by teleport-update before their are linked into system locations.

The following conditions will result in a warning and skip the file:

• Irregular file
• Non-executable regular file
• Shell script

The following conditions will error:

• Unreadable file
• Executable binary that cannot execute on the host (run with version as argv[1])

This makes teleport-update resilient to unknown non-binary files that may be present in the teleport tgz in past or future releases (e.g., other install scripts).

Example:

ubuntu@legendary-mite:~$ sudo ./teleport-update enable --force-version=16.4.7
2024-12-04T06:37:53Z INFO [UPDATER]   Initiating installation. target_version:16.4.7 active_version: agent/updater.go:304
2024-12-04T06:37:54Z INFO [UPDATER]   Downloading Teleport tarball. url:https://cdn.teleport.dev/teleport-ent-v16.4.7-linux-arm64-bin.tar.gz size:162964120 agent/installer.go:324
2024-12-04T06:38:00Z INFO [UPDATER]   Extracting Teleport tarball. path:/opt/teleport/default/versions/16.4.7 size:648448000 agent/installer.go:362
2024-12-04T06:38:03Z INFO [UPDATER]   Validating binary name:fdpass-teleport agent/validate.go:68
2024-12-04T06:38:03Z INFO [UPDATER]   Binary does not support version command name:fdpass-teleport agent/validate.go:79
2024-12-04T06:38:03Z INFO [UPDATER]   Validating binary name:tbot agent/validate.go:68
2024-12-04T06:38:03Z INFO [UPDATER]   [stdout] Teleport v16.4.7 git:v16.4.7-0-g15dfef1 go1.22.9 agent/process.go:385
2024-12-04T06:38:03Z INFO [UPDATER]   Validating binary name:tctl agent/validate.go:68
2024-12-04T06:38:03Z INFO [UPDATER]   [stdout] Teleport v16.4.7 git:v16.4.7-0-g15dfef1 go1.22.9 agent/process.go:385
2024-12-04T06:38:03Z INFO [UPDATER]   Validating binary name:teleport agent/validate.go:68
2024-12-04T06:38:03Z INFO [UPDATER]   [stdout] Teleport Enterprise v16.4.7 git:v16.4.7-0-g15dfef1 go1.22.9 agent/process.go:385
2024-12-04T06:38:03Z INFO [UPDATER]   Validating binary name:tsh agent/validate.go:68
2024-12-04T06:38:03Z INFO [UPDATER]   [stdout] Teleport v16.4.7 git:v16.4.7-0-g15dfef1 go1.22.9 agent/process.go:385
2024-12-04T06:38:03Z INFO [UPDATER]   Executing new teleport-update binary to update configuration. agent/updater.go:133
2024-12-04T06:38:04Z INFO [UPDATER]   Systemd configuration synced. unit:teleport-update.timer agent/process.go:259
2024-12-04T06:38:04Z INFO [UPDATER]   Service enabled. unit:teleport-update.timer agent/process.go:276
2024-12-04T06:38:04Z INFO [UPDATER]   Finished executing new teleport-update binary. agent/updater.go:139
2024-12-04T06:38:04Z INFO [UPDATER]   Target version successfully installed. target_version:16.4.7 agent/updater.go:676
2024-12-04T06:38:05Z INFO [UPDATER]   Gracefully reloaded. unit:teleport.service agent/process.go:110
2024-12-04T06:38:05Z INFO [UPDATER]   Monitoring PID file to detect crashes. unit:teleport.service agent/process.go:113
2024-12-04T06:38:19Z INFO [UPDATER]   Configuration updated. agent/updater.go:320

---

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 /opt/teleport.

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

[vapopov]

LGTM