net/netmon, ipn/ipnlocal, wgengine: don't lose a network change during startup

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If the network comes up during the few milliseconds NewLocalBackend
takes, tailscaled starts with its control client paused and never
unpauses: the interface state snapshot was taken before the netmon
subscription (kept late since #17252), so a change published in between
reached nobody. #21261 hit it on fast-booting NixOS microVMs, and it was
behind the natlab TestEasyEasy CI flake, where a gokrazy node's DHCP
lease landed in that window and "tailscale up" hung at "awaiting
unpause".

Re-read netmon's state after subscribing rather than subscribing before
the snapshot (as #21281 proposed), which would bring back the #17252
data race and let a fresh delta be overwritten by the stale snapshot.
netmon.New and the userspace engine had the same shape of gap between
their snapshot and their subscription; close those too.

Under CPU pressure the hang reproduced in 1 of 22 TestEasyEasy runs
before and 0 of 120 after.

Thanks to @c-vigo for the detailed diagnosis in #21261 and to @zzz-yu
for pointing out this problem and proposing a fix in #21281!

Fixes #21261
Updates #14902
Updates #19126
Updates #deflake

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: Ifa21f8055a85321a5afda7800140fc5045c6ce08

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