ipn/ipnserver: enable systemd-notify support

Addresses #964

Still to be done:
- Figure out the correct logging lines in util/systemd
- Figure out if we need to slip the systemd.Status function anywhere
  else
- Log util/systemd errors? (most of the errors are of the "you cannot do
  anything about this, but it might be a bad idea to crash the program if
  it errors" kind)

Assistance in getting this over the finish line would help a lot.

Signed-off-by: Christine Dodrill <me@christine.website>

util/systemd: rename the nonlinux file to appease the magic

Signed-off-by: Christine Dodrill <me@christine.website>

util/systemd: fix package name

Signed-off-by: Christine Dodrill <me@christine.website>

util/systemd: fix review feedback from @mdlayher

Signed-off-by: Christine Dodrill <me@christine.website>

cmd/tailscale{,d}: update depaware manifests

Signed-off-by: Christine Dodrill <me@christine.website>

util/systemd: use sync.Once instead of func init

Signed-off-by: Christine Dodrill <me@christine.website>

control/controlclient: minor review feedback fixes

Signed-off-by: Christine Dodrill <me@christine.website>

{control,ipn,systemd}: fix review feedback

Signed-off-by: Christine Dodrill <me@christine.website>

review feedback fixes

Signed-off-by: Christine Dodrill <me@christine.website>

ipn: fix sprintf call

Signed-off-by: Christine Dodrill <me@christine.website>

ipn: make staticcheck less sad

Signed-off-by: Christine Dodrill <me@christine.website>

ipn: print IP address in connected status

Signed-off-by: Christine Dodrill <me@christine.website>

ipn: review feedback

Signed-off-by: Christine Dodrill <me@christine.website>

final fixups

Signed-off-by: Christine Dodrill <me@christine.website>

ipn/ipnserver/server.go

@@ -34,6 +34,7 @@ import (
 	"tailscale.com/smallzstd"
 	"tailscale.com/types/logger"
 	"tailscale.com/util/pidowner"
+	"tailscale.com/util/systemd"
 	"tailscale.com/version"
 	"tailscale.com/wgengine"
 )
@@ -589,6 +590,7 @@ func Run(ctx context.Context, logf logger.Logf, logid string, getEngine func() (
 		})
 	}
 
+	systemd.Ready()
 	for i := 1; ctx.Err() == nil; i++ {
 		var c net.Conn
 		var err error