Fix compiling on GNU/Hurd. [skip appveyor]

GNU/Hurd does not define MAXHOSTNAMELEN because it does not define
HOST_NAME_MAX, even if __USE_POSIX is defined.  This causes the
following:

./print-bgp.c:1997:14: error: 'MAXHOSTNAMELEN' undeclared (first use in
  this function)
./print-bgp.c:2965:14: error: 'MAXHOSTNAMELEN' undeclared (first use in
  this function)
./print-icmp.c:502:11: error: 'MAXHOSTNAMELEN' undeclared (first use in
  this function)

However, the above code should not depend on this OS-specific detail to
size its temporary printing buffers, so just hard-code the sizes.

Author: infrastation

CHANGES

@@ -40,6 +40,7 @@ DayOfTheWeek, Month DD, YYYY / The Tcpdump Group
       Detect OS IPv6 support using AF_INET6 only.
       Remove all remaining support for BSD/OS, IRIX, {OSF/1, Digital Unix,
         Tru64 Unix}, SINIX and Ultrix.
+      Fix compiling on GNU/Hurd.
 
 DayOfTheWeek, Month DD, YYYY / The Tcpdump Group
   Summary for 4.99.5 tcpdump release (so far!)

README.md

@@ -31,6 +31,7 @@ tcpdump compiles and works on at least the following platforms:
 * Haiku
 * HP-UX 11i
 * illumos (OmniOS, OpenIndiana)
+* GNU/Hurd
 * GNU/Linux
 * {Mac} OS X / macOS
 * NetBSD

print-bgp.c

@@ -1994,7 +1994,7 @@ bgp_attr_print(netdissect_options *ndo,
     int advance;
     u_int tlen;
     const u_char *tptr;
-    char buf[MAXHOSTNAMELEN + 100];
+    char buf[512];
     u_int as_size;
     int add_path4, add_path6;
     int ret;
@@ -2962,7 +2962,7 @@ bgp_update_print(netdissect_options *ndo,
 {
     const u_char *p;
     u_int withdrawn_routes_len;
-    char buf[MAXHOSTNAMELEN + 100];
+    char buf[512];
     int wpfx;
     u_int len;
     int i;

print-icmp.c

@@ -499,7 +499,7 @@ icmp_print(netdissect_options *ndo, const u_char *bp, u_int plen,
 	const char *str;
 	const uint8_t *obj_tptr;
 	u_int hlen;
-	char buf[MAXHOSTNAMELEN + 100];
+	char buf[512];
 	struct cksum_vec vec[1];
 
 	ndo->ndo_protocol = "icmp";