cmd/tailscaled: support "ts_omit_ssh" build tag to remove SSH

Some environments would like to remove Tailscale SSH support for the
binary for various reasons when not needed (either for peace of mind,
or the ~1MB of binary space savings).

Updates tailscale/corp#24454
Updates #1278
Updates #12614

Change-Id: Iadd6c5a393992c254b5dc9aa9a526916f96fd07a
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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