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>

control/controlclient/direct.go

@@ -44,6 +44,7 @@ import (
 	"tailscale.com/types/logger"
 	"tailscale.com/types/opt"
 	"tailscale.com/types/structs"
+	"tailscale.com/util/systemd"
 	"tailscale.com/version"
 )
 
@@ -308,6 +309,7 @@ func (c *Direct) doLoginOrRegen(ctx context.Context, t *oauth2.Token, flags Logi
 	if mustregen {
 		_, url, err = c.doLogin(ctx, t, flags, true, url)
 	}
+
 	return url, err
 }
 
@@ -328,6 +330,7 @@ func (c *Direct) doLogin(ctx context.Context, t *oauth2.Token, flags LoginFlags,
 
 	if expired {
 		c.logf("Old key expired -> regen=true")
+		systemd.Status("key expired; run 'tailscale up' to authenticate")
 		regen = true
 	}
 	if (flags & LoginInteractive) != 0 {