This adds a new `UserLogf` field to the `Server` struct.
When set this any logs generated by Server are logged using
`UserLogf` and all spammy backend logs are logged to `Logf`.
If it `UserLogf` is unset, we default to `log.Printf` and
if `Logf` is unset we discard all the spammy logs.
Fixes#12094
Signed-off-by: Maisem Ali <maisem@tailscale.com>