dhcp: support option 52 overload

[ssam18]

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Link to ticket: https://redmine.openinfosecfoundation.org/issues/8538

Per RFC 2132 the BOOTP sname and file fields can hold extra DHCP options when the standard options area carries Option Overload (52), but the parser ignored those continuation areas. This closes the visibility gap reported in 8538 where attacker controlled dns_servers, routers or domain values placed inside sname stayed invisible to EVE logging and detection. End to end testing on the reporter pcap shows the overloaded OFFER and ACK now surfacing dns_servers 10.100.0.2 and routers 10.100.0.2 in eve.json, while the parallel benign flow in the same capture continues to report its inline values unchanged.

Replaces #15340.

Describe changes:

• After parsing the main DHCP options, look up Option Overload (code 52) and walk the BOOTP file or sname continuation area as additional option streams, appending the parsed entries to the same options vector.
• Add eight unit tests covering all three overload values, malformed continuation, no overload, and the actual pcap from issue 8538.
• Document the new behavior in the EVE DHCP section of the user guide.

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