lib, watchfrr: Add some additional status messages to systemd

lib, watchfrr: Add some additional status messages to systemd

Allow systemd to be informed about operational state so operators can
infer a bit about what is going on with FRR from the systemd status
cli.

sharpd@robot ~/frr4> systemctl status frr
● frr.service - FRRouting
   Loaded: loaded (/usr/lib/systemd/system/frr.service; enabled; vendor preset: disabled)
   Active: active (running) since Thu 2019-10-03 21:09:04 EDT; 7s ago
     Docs: https://frrouting.readthedocs.io/en/latest/setup.html
  Process: 32455 ExecStart=/usr/lib/frr/frrinit.sh start (code=exited, status=0/SUCCESS)
   Status: "FRR Operational"
    Tasks: 12 (limit: 4915)
   Memory: 76.5M
   CGroup: /system.slice/frr.service
           ├─32468 /usr/lib/frr/watchfrr -d zebra bgpd staticd
           ├─32487 /usr/lib/frr/zebra -d -A 127.0.0.1 -s 90000000
           ├─32492 /usr/lib/frr/bgpd -d -A 127.0.0.1
           └─32500 /usr/lib/frr/staticd -d -A 127.0.0.1

Please note the `Status: ...` line above.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>