networkctl: add dhcp-lease verb to dump DHCP lease

This shows the DHCP message of an acquired DHCP lease.
Example:
```
$ networkctl dhcp-lease eth0
Header:
              KEY VALUE
   Hardware Type: ETHER
Hardware Address: 41:42:43:31:32:33
  Client Address: 192.0.2.123
  Server Address: 192.0.2.1

Options:
CODE NAME               DATA
   1 subnet mask        255.255.255.0
   3 router             192.0.2.1
   6 domain name server 192.0.2.1
  12 hostname           test-node
  15 domain name        lan
  28 broadcast address  192.0.2.255
  42 NTP server         192.0.2.11
                        192.0.2.12
  51 lease time         1d
  53 message type       5
  54 server identifier  192.0.2.1
  58 renewal time       11h 5min 38s
  59 rebinding time     20h 5min 38s
 119 domain search      hoge.example.com
                        foo.example.com
```

shell-completion/bash/networkctl

@@ -51,7 +51,7 @@ _networkctl() {
 
     local -A VERBS=(
         [STANDALONE]='label reload'
-        [LINKS]='status list lldp delete renew up down forcerenew reconfigure'
+        [LINKS]='status dhcp-lease list lldp delete renew up down forcerenew reconfigure'
         [FILES_OR_LINKS]='edit cat'
         [FILES]='mask unmask'
         [BOOL]='persistent-storage'