sockstat: Add a -f option to show FIB numbers of sockets

[markj]

Summary

Sponsored by: Klara, Inc.
Sponsored by: Stormshield

[zlei]

usr.bin/sockstat/sockstat.c

1342

By default the GENERIC kernel limits the max number of fibs to 65535, See sys/net/route/route_var.h:

#define	RT_MAXFIBS	UINT16_MAX

So 6 chars width should be sufficient enough. But it would be better if you bother to explain this magic number :)