envknob: add CanTaildrop (TS_DISABLE_TAILDROP) to disable taildrop on device

This matches CanSSHD (TS_DISABLE_SSH_SERVER) for administratively
disabling the code on a node, regardless of local or server configs.

This can be configured in /etc/default/tailscaled on Linux,
%ProgramData%\Tailscale\tailscaled-env.txt on Windows,
or /etc/tailscale/tailscaled-env.txt on Synology. (see getPlatformEnvFile)

Also delete some dead code and tidy up some docs.

Change-Id: I79a87c03e33209619466ea8aeb0f6651afcb8789
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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