paths, version/distro: detect Synology DSM version better, use for socket path

Resolves a TODO in the code noted while discussing QNAP defaults.

Tested on DSM6 and DSM7.

Change-Id: Icce03ff41fafd7b3a358cfee16f2ed13d5cc3c5d
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f