ipn/ipnlocal: use more buildfeatures consts so the linker drops dead code

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Several features with ts_omit build tags were still linked into
minimal builds because LocalBackend called into them unconditionally,
so the linker couldn't prove them unreachable. Guard those call sites
with their buildfeatures constants:

  * app connectors: only subscribe to route update and store events
    when app connectors are included; appc is their only publisher.
    This also drops AdvertiseRoute and UnadvertiseRoute.
  * client update: only subscribe to the tailnet default auto-update
    event when auto-updates are supported, as that's all it affects.
  * syspolicy: only register the policy change watch when system
    policy support is included.
  * advertise routes and exit nodes: only validate AdvertiseRoutes
    when one of the two features is included.
  * netstack: only netstack consults the intercepted TCP ports, so
    don't build the port matching func without it.
  * ssh: don't check SSH prefs, report the "SSH on but unusable"
    health message, or intercept port 22 without SSH support. The
    RunSSH pref is still accepted without effect, as before; port 22
    was previously intercepted by netstack builds without SSH.
  * web client: don't track whether the web client should run when
    it's not included.
  * tailnet lock: peers marked UnsignedPeerAPIOnly are only special
    in that tailnet lock skips their signature checks, and the packet
    filter check guards that exemption. Without tailnet lock, all
    peers from control are trusted anyway, so skip the check.

This reduces the size of a build_dist.sh --min tailscaled binary on
linux/amd64 by 65,536 bytes (0.5%), about 28 KB of it in ipnlocal and
most of the rest in eventbus and slices generic instantiations for the
events no longer subscribed to.

Updates #12614

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: I8f4d4c0779e84f46f3b076d0fb35514c69a52d8f

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