netstat: Add -F support for -g

This change adds the ability to examine the contents of multicast
routing tables for other FIBs without the need for executing
`netstat` with `setfib(1)`.

MFC after:	3 days
Sponsored by:	Klara, Inc.
Sponsored by:	Stormshield
Differential Revision:	https://reviews.freebsd.org/D56205
Reviewed by:	glebius, markj, zlei

usr.bin/netstat/main.c

@@ -550,6 +550,8 @@ main(int argc, char *argv[])
 
 
 	if (gflag) {
+		if (fib != -1 && setfib(fib) < 0)
+			xo_errx(EX_DATAERR, "setfib: %s", strerror(errno));
 		xo_open_container("statistics");
 		xo_set_version(NETSTAT_XO_VERSION);
 		if (sflag) {

usr.bin/netstat/netstat.1

@@ -25,7 +25,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd April 3, 2026
+.Dd April 7, 2026
 .Dt NETSTAT 1
 .Os
 .Sh NAME
@@ -94,6 +94,7 @@
 .Op Fl j Ar jail
 .Op Fl -libxo
 .Op Fl 46W
+.Op Fl F Ar fibnum
 .Op Fl f Ar address_family
 .It Nm Fl gs
 .Op Fl j Ar jail
@@ -738,6 +739,7 @@ See
 .Nm
 .Fl g
 .Op Fl 46W
+.Op Fl F Ar fibnum
 .Op Fl f Ar address_family
 .Op Fl M Ar core
 .Op Fl N Ar system
@@ -764,6 +766,15 @@ See
 .Sx GENERAL OPTIONS .
 .It Fl W
 Avoid truncating addresses even if this causes some fields to overflow.
+.It Fl F
+Display the routing table with the number
+.Ar fibnum .
+If the specified
+.Ar fibnum
+is -1 or
+.Fl F
+is not specified,
+the default routing table is displayed.
 .It Fl f Ar protocol_family
 Filter by
 .Ar protocol_family .