Related to #18009 - Have randpkt default to pcapng, allow selecting
a different capture file format via the common -F option that other
command line tools use, and document it.
For the randpktdump extcap, just use pcapng.
This fixes --all-random, because --all-random requires different
encapsulation per packet. It also fixes the related -r option to
randpkt (though note that picking a file format that doesn't support
ENCAP_PER_PACKET with -r causes problems.)
Document -r in the randpkt man page.
Fix#18944