teach tcpdump about gre-in-udp encapsulation

[David Gwynne]

this is the simplest diff i could make that adds gre in udp handling
to tcpdump.

ok?

Index: print-udp.c
===================================================================
RCS file: /cvs/src/usr.sbin/tcpdump/print-udp.c,v
retrieving revision 1.40
diff -u -p -r1.40 print-udp.c
--- print-udp.c 22 Dec 2015 21:01:07 -0000      1.40
+++ print-udp.c 3 Jul 2018 02:48:13 -0000
@@ -374,6 +374,7 @@ static int udp6_cksum(const struct ip6_h
 #define GTP_U_PORT             2152
 #define GTP_PRIME_PORT         3386
 #define UDPENCAP_PORT          4500            /*XXX*/
+#define GRE_PORT               4754
 #define MULTICASTDNS_PORT      5353
 
 #ifdef INET6
@@ -640,6 +641,10 @@ udp_print(const u_char *bp, u_int length
                        iapp_print((const u_char *)(up + 1), length);
                else if (ISPORT(VQP_PORT))
                        vqp_print((const u_char *)(up + 1), length);
+               else if (ISPORT(GRE_PORT)) {
+                       printf(" ");
+                       gre_print((const u_char *)(up + 1), length);
+               }
 #ifdef INET6
                else if (ISPORT(RIPNG_PORT))
                        ripng_print((const u_char *)(up + 1), length);