tsnet: reset serve config only once

Prior to this change, we were resetting the tsnet's serve config every
time tsnet.Server.Up was run. This is important to do on startup, to
prevent messy interactions with stale configuration when the code has
changed.

However, Up is frequently run as a just-in-case step (for example, by
Server.ListenTLS/ListenFunnel and possibly by consumers of tsnet). When
the serve config is reset on each of these calls to Up, this creates
situations in which the serve config disappears unexpectedly. The
solution is to reset the serve config only on the first call to Up.

Fixes #8800
Updates tailscale/corp#27200
Signed-off-by: Harry Harpham <harry@tailscale.com>

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