Make sure "isc-dhcp-server" starts successfully

[jcapona]

Sometimes isc-dhcp-server.service will report it started successfully, but going through the logs, it will report errors that need to be fixed. Otherwise, the dhcp server won't lease IPs.

pi@pi-top:~ $ systemctl status isc-dhcp-server
● isc-dhcp-server.service - LSB: DHCP server
   Loaded: loaded (/etc/init.d/isc-dhcp-server; generated)
   Active: active (running) since Tue 2021-07-13 19:59:10 EDT; 2min 4s ago
     Docs: man:systemd-sysv-generator(8)
    Tasks: 2 (limit: 3737)
   CGroup: /system.slice/isc-dhcp-server.service
           ├─ 798 /usr/sbin/dhcpd -4 -q -cf /etc/dhcp/dhcpd.conf ptusb0
           └─1026 /usr/sbin/dhcpd -4 -q -cf /etc/dhcp/dhcpd.conf ptusb0 wlan_ap0

Jul 13 19:59:07 pi-top dhcpd[1026]: 
Jul 13 19:59:07 pi-top dhcpd[1026]: No subnet declaration for wlan_ap0 (no IPv4 
Jul 13 19:59:07 pi-top dhcpd[1026]: ** Ignoring requests on wlan_ap0.  If this i
Jul 13 19:59:07 pi-top dhcpd[1026]:    you want, please write a subnet declarati
Jul 13 19:59:07 pi-top dhcpd[1026]:    in your dhcpd.conf file for the network s
Jul 13 19:59:07 pi-top dhcpd[1026]:    to which interface wlan_ap0 is attached. 
Jul 13 19:59:07 pi-top dhcpd[1026]: 
Jul 13 19:59:07 pi-top dhcpd[1026]: Server starting service.
Jul 13 19:59:10 pi-top isc-dhcp-server[989]: Starting ISC DHCPv4 server: dhcpd.
Jul 13 19:59:10 pi-top systemd[1]: Started LSB: DHCP server.

wifi-ap-sta needs to make sure that this service is actually started for it to work properly.