Add capabilities check for feature dhcp-discover

[DL6ER]

What does this implement/fix?

Add capabilities check for feature pihole-FTL dhcp-discover in the same way we already have it for pihole-FTL arp-scan

Related issue or feature (if applicable): N/A

Pull request in docs with documentation (if applicable): N/A

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[yubiuser]

Maybe you could tweak the failure output a bit (different PR): e.g. which interface failed? I guess it's my docker interfaces and the wireguard interface.

Scanning all your interfaces for DHCP servers
Timeout: 10 seconds

WARN: Could not sendto() in send_dhcp_discover() (/__w/FTL/FTL/src/tools/dhcp-discover.c:235): Operation not permitted
WARN: Could not sendto() in send_dhcp_discover() (/__w/FTL/FTL/src/tools/dhcp-discover.c:235): Operation not permitted
* Received 548 bytes from eno1:10.0.1.1

[yubiuser]

Maybe it's only wireguard.

DHCP packets received on interface eno1: 2
DHCP packets received on interface lo: 0
DHCP packets received on interface br-8f8a772de3d9: 0
DHCP packets received on interface docker0: 0
DHCP packets received on interface veth57f8f05: 0
DHCP packets received on interface vethee977de: 0
DHCP packets received on interface vetha384b51: 0
DHCP packets received on interface br-a638779dc7e3: 0