tsweb/varz: add node_boot_time_seconds expvar

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Export the machine's boot time (the btime line from Linux's
/proc/stat) as node_boot_time_seconds, named to match what
Prometheus's node exporter uses for the same value. Combined with
process_start_unix_time, this can be used to distinguish process
restarts from whole node restarts.

The value is parsed once per process lifetime, not per scrape, and
the metric is only published when a value is available, so non-Linux
systems don't export a bogus zero.

Updates tailscale/corp#44743

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: I5f53186b97bb1482bd1a5387c0910b0ae26544ff

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