hostinfo, tailcfg: split Hostinfo.OSVersion into separate fields

Stop jamming everything into one string.

Fixes #5578

Change-Id: I7dec8d6c073bddc7dc5f653e3baf2b4bf6b68378
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f